In this article, we discuss conditions that are sufficient for the existence of solutions for some ψ-Hilfer fractional integro-differential equations with non-instantaneous impulsive multi-point boundary conditions. By applying Krasnoselskii's and Banach's fixed point theorems, we investigate the existence and uniqueness of these solutions. Moreover, we have proved its boundedness of the method. We extend some earlier results by introducing and including the ψ-Hilfer fractional derivative, nonlinear integral terms and non-instantaneous impulsive conditions. Finally, we offer an application to explain the consistency of our theoretical results.
Citation: Thabet Abdeljawad, Pshtiwan Othman Mohammed, Hari Mohan Srivastava, Eman Al-Sarairah, Artion Kashuri, Kamsing Nonlaopon. Some novel existence and uniqueness results for the Hilfer fractional integro-differential equations with non-instantaneous impulsive multi-point boundary conditions and their application[J]. AIMS Mathematics, 2023, 8(2): 3469-3483. doi: 10.3934/math.2023177
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In this article, we discuss conditions that are sufficient for the existence of solutions for some ψ-Hilfer fractional integro-differential equations with non-instantaneous impulsive multi-point boundary conditions. By applying Krasnoselskii's and Banach's fixed point theorems, we investigate the existence and uniqueness of these solutions. Moreover, we have proved its boundedness of the method. We extend some earlier results by introducing and including the ψ-Hilfer fractional derivative, nonlinear integral terms and non-instantaneous impulsive conditions. Finally, we offer an application to explain the consistency of our theoretical results.
As a generalization of cyclic codes and negacyclic codes, constacyclic codes were first introduced by Berlekamp in 1968 [3]. Given a nonzero element λ in a finite filed Fq, a linear code C of length n over Fq is called λ-constacyclic if (λcn−1,c0,⋯,cn−2)∈C for every (c0,c1,⋯,cn−1)∈C. Constacyclic codes over finite fields form a remarkable class of linear codes, as it includes the class of cyclic codes and the class of negacyclic codes as proper subclasses. Constacyclic codes have rich algebraic structure so that they can be efficiently encoded and decoded by means of shift registers. Repeated-root constacyclic codes were a special class of constacyclic codes. Repeated-root constacyclic codes were first studied by Castagnoli et al. [4] and van Lint [13], and they showed that repeated-root cyclic codes have a concatenated construction and are not asymptotically good.
Recently, repeated-root constacyclic codes have been studied by many authors. To determine the generator polynomials of all constacyclic codes of arbitrary length over finite fields is an important problem. Dinh studied repeated-root constacyclic codes of lengths 2ps, 3ps, 4ps and 6ps in a series of papers [8,9,10,11]. He determined the algebraic structure of these repeated-root constacyclic codes over finite fields in terms of their generator polynomials. In [7], Chen et al. introduced an equivalence relation called isometry for the nonzero elements of Fq to classify constacyclic codes of length n over Fq. They have the same distance structures and the same algebraic structures for belonging to the same equivalence classes induced by isometry. Furthermore, in [5], Chen et al. considered a more specified relationship than isometry that enabled us to obtain more explicit description of generator polynomials of all constacyclic codes. According to the equivalence classes, all constacyclic codes of length ℓps over Fqm and their dual are characterized, where ℓ is a prime different from p and s is a positive integer. In 2012, Bakshi and Raka [1] also determined all Λ-constacyclic codes of length 2tps(t≥1,s≥0 are integers) over Fpr using different methods from Chen et al.. In 2015, Chen et al. [6] determined the algebraic structure of all constacyclic codes of length 2ℓmps over Fpr and their dual codes in terms of their generator polynomials, where ℓ,p are distinct odd primes and s,m are positive integers. In the conclusion of the paper [6], they proposed an open problem to study all constacyclic codes of length kℓmps over Fq, where p is the characteristic of Fq, ℓ is an odd prime different from p, and k is a prime different from ℓ and p. Batoul et al. [2] investigated the structure of constacyclic codes of length 2ampr over Fps with a≥1 and (m,p)=1. They also provided certain sufficient conditions under which these codes are equivalent to cyclic codes of length 2ampr over Fps. Sharma [16] determined all constacyclic codes of length ℓtps over Fpr and their dual codes, where ℓ,p are distinct primes, ℓ is odd and s,t,r are positive integers. In 2016, Sharma et al. [17] determine generator polynomials of all constacyclic codes of length 4ℓmpn over the finite field Fq and their dual codes, where p,ℓ are distinct odd primes, q is a power of p and m,n are positive integers. Working in the same direction, Liu et al. obtained generator polynomials of all repeated-root constacyclic codes of length 3ℓps over Fq in [14], where ℓ is an odd prime different from p and 3. In 2017, Liu et al. [15] explicitly determine the generator polynomials of all repeated-root constacyclic codes of length nℓps over Fq and their dual codes, where ℓ is an odd prime different from p, and n is an odd prime different from both ℓ and p such that n=2h+1 for some prime h. In 2019, Wu and Yue et al. [19,20] explicitly factorize the polynomial xn−λ for each λ∈Fq. As applications, they obtain all repeated-root λ-constacyclic codes and their dual codes of length nps over Fq.
In this paper, we answer the question of B. Chen, H. Dinh and Liu. That is we determine all the constacyclic codes of length p1pt2ps over Fq, where p is the characteristic of Fq, p1 is an odd prime different from p, and p1 is a prime different from p2 and p. We give the explicit generator polynomials of all the constacyclic codes of length p1pt2ps over Fq and their dual codes, and determine all self-dual cyclic codes of length p1pt2ps and their enumeration.
The remainder of this paper is organized as follows. In Section 2 we give a brief background on some basic results which we need in the following parts. In Section 3, we calculate the q-cyclotomic cosets modulo p1pt2 as a preparation for giving the generator polynomials of constacyclic codes of length p1pt2ps over Fq. In Section 4, we first describe a general method to obtain the generator polynomials of constacyclic codes, and then with this method and the results of q-cyclotomic cosets modulo p1pt2 we give the explicit generator polynomials of all the constacyclic codes of length p1pt2ps. And in Section 5, all the self-dual cyclic codes of length p1pt2ps over Fq are given. In the last section, as an example we calculate the case of length 5ℓps, where ℓ is a prime different from 5 and p.
In this section, we first review some basic results in number theory and finite fields, which we will in the following parts, and then give a brief introduction to the λ-constacyclic codes. For a positive integer n, we denote by Zn the ring of integers module n throughout this paper. Let p be a prime number, and q be a power of p. We denote by Fq the finite field with q elements, and fix a generator element ξ of the multiplicative group F∗q, that is, F∗q=⟨ξ⟩. In this paper, we mainly deal with the repeated-root constacyclic codes of length p1pt2ps over Fq, where p1 and p2 are two distinct odd prime numbers different from p. For any positive integer d and i=1,2, we write fi,d=ordpdi(q) for the multiplicative order of q modulo pdi, and set gi,d=ϕ(pdi)fi,d, where ϕ is the Euler's phi function. When d=1, we write fi=fi,1 and gi=gi,1 for simplicity. For i=1,2, there are positive integers ui and wi such that qfi=1+puiiwi and pi∤wi. Following the lifting-the-exponent lemma, we immediately have
fi,d=fipmax{0,d−ui}i. |
Lemma 2.1. [12] Assume that r is a primitive root of the odd prime p and (r+tp)p−1 is not congruent to 1 modulo p2. Then r+tp is a primitive root of pk for each k≥1.
Lemma 2.2. [18] Let n≥2 be an integer, and λ be a nonzero element in Fq with multiplicative order k=ord(λ). The binomial xn−λ is irreducible over Fq if and only if
(1) Every prime divisor of n divides k, but not q−1k;
(2) If 4∣n, then 4∣(q−1).
Let λ be a nonzero element in Fq. A λ-constacyclic code of length n is a linear code C such that (c0,c1,⋯,cn−1)∈C implies (λcn−1,c0,⋯,cn−2)∈C. This definition is a natural generalization of cyclic code and negacyclic code. A λ-constacyclic code C of length n over Fq can be regarded as an ideal (g(x)) of the quotient ring Fq[x]/(xn−λ), where g(x) is a divisor of xn−λ. Let C be a λ-constacyclic code of length n over Fq, then the dual code of code C is given by C⊥={x∈Fnq:x⋅y=0,∀y∈C}, where x⋅y denotes the Euclidean inner product of x and y. If C is generated by a polynomial g(x) satisfying g(x)∣xn−λ, and h(x) is given by h(x)=xn−λg(x), then h(x) is called the parity check polynomial of code C. It is a classical result that the dual code C⊥ is generated by h(x)∗, where h(x)∗=h(0)−1xdeg(h(x))h(x−1) is the reciprocal polynomial of h(x). The code C is called to be a self-orthogonal if C⊆C⊥ and a self-dual code if C=C⊥. For self-dual cyclic code, a well-known result states that there exist self-dual cyclic codes of length n over Fq if and only if n is even and the characteristic of Fq is p=2.
There are q−1 classes of constacyclic codes of length n over Fq. However, some of them are turned out to be equivalent in the sense that they have the same structure. To be explicit, two elements λ,μ∈F∗q are called n-equivalent in F∗qif there exists a∈F∗q such that anλ=μ.
Lemma 2.3. [5] For any λ,μ∈F∗q, the following four statements are equivalent:
(1) λ and μ are n-equivalent in F∗q.
(2) λ−1μ∈⟨ξn⟩.
(3) (λ−1μ)d=1, where d=q−1gcd(n,q−1).
(4) There exists an a∈F∗q such that
φa:Fq[X]/(Xn−μ)→Fq[X]/(Xn−λ);f(X)↦f(aX) |
is an Fq-algebra isomorphism. In particular, there are gcd(n,q−1) n-equivalence classes in F∗q.
We conclude this section with the introduction of q-cyclotomic coset which is important in the computation of constacyclic codes. Let n be a positive integer relatively prime to n. For 0≤s≤n−1, the q-cyclotomic coset of s modulo n is defined to be
Cs={s,sq,⋯,sqns−1}, |
where ns is the least positive integer such that sq^{n_{s}}\equiv s \pmod n . It is obvious to see that n_{s} is equal to the multiplicative order of q modulo \frac{n}{\gcd(s, n)} . Notice that if sq^{a} \equiv s^{\prime}q^{b} \pmod n for some positive integers a, b , then
s \equiv sq^{a+(n_{s}-a)} \equiv s^{\prime}q^{b+(n_{s}-a)} \pmod n. |
It follows that for 0 \leq s, s^{\prime} \leq n-1 , C_{s} \cap C_{s^{\prime}} \neq \emptyset if and only if C_{s} = C_{s^{\prime}} . Therefore the q -cyclotomic cosets give a classification of the element in \mathbb{Z}_{n} .
If \alpha is a primitive n th root of unit in some extension field of \mathbb{F}_q , then the polynomial
C_{s}(x) = \prod\limits_{i\in C_{s}}(x-\alpha^{i}) |
is exactly the minimal polynomial of \alpha^{s} over \mathbb{F}_q , and
x^{n}-1 = \prod\limits_{s} C_{s}(x) |
gives the irreducible factorization of x^{n}-1 over \mathbb{F}_q , where s runs over all representations of distinct q -cyclotomic cosets modulo n . We call C_{s}(x) the polynomial associated to C_{s} .
Let C_{s} = \{s, sq, \cdots, sq^{n_{s}-1}\} be any q -cyclotomic coset modulo n . The reciprocal coset of C_{s} is defined to be
C_{s}^{*} = \{-s, -sq, \cdots, -sq^{n_{s}-1}\}. |
We say that the coset C_{s} is self-reciprocal if C_{s} = C_{s}^{*} . One can check that the polynomial C_{s}^{*}(x) associated to the reciprocal coset C_{s}^{*} is exactly the reciprocal polynomial of C_{s}(x) .
The q -cyclotomic cosets modulo p_{1}p_{2}^{t} plays an important role in determining all the constacyclic codes of length p_{1}p_{2}^{t}p^{s} . In this section we consider a more general case that classifies all the q -cyclotomic cosets modulo p_{1}^{t_{1}}p_{2}^{t_{2}} , where p_{1} and p_{2} are two distinct odd prime numbers not dividing q , and t_{1}, t_{2} are positive integers.
Let \ell be a prime number not dividing q , and \mu be a generator of the cyclic group \mathbb{Z}_{\ell}^{*} . It is obvious that all the q -cyclotomic cosets modulo \ell are given by C_{0} = \{0\} and
C_{k} = \{\mu^{k}, \mu^{k}q, \cdots, \mu^{k}q^{\mathrm{ord}_{\ell}(q)-1}\}, \ 1\leq k\leq \dfrac{\ell - 1}{\mathrm{ord}_{\ell}(q)}. |
For different odd prime numbers p_{1} and p_{2} , we claim that there exists an integer \mu_{1} satisfying that:
(1) \mu_{1} is a primitive root modulo p_{1}^{d} for all d\geq 1 ; and
(2) \mu_{1}\equiv 1 \pmod {p_{2}} .
We begin with a random primitive root \eta_{1}^{'} modulo p_{1} . If p_{1}^{2}\nmid {\eta_{1}^{'}}^{p_{1}-1}-1 , we let \eta_{1} = \eta_{1}^{'} , otherwise we let \eta_{1} = \eta_{1}^{'}+p_{1} . It is trivial to see that \eta_{1} satisfies the condition \gcd(\frac{\eta_{1}^{p_{1}-1}-1}{p_{1}}, p_{1}) = 1 . Let \mu_{1} = \eta_{1}+(1-\eta_{1})p_{1}^{p_{2}-1} , then
\mu_{1}^{p_{1}-1}-1\equiv (\eta_{1}+(1-\eta_{1})p_{1}^{p_{2}-1})^{p_{1}-1}-1\equiv \eta_{1}^{p_{1}-1}-1 \pmod {p_1^2}. |
It follows that
\gcd(\frac{\mu_{1}^{p_{1}-1}-1}{p_{1}}, p_{1}) = \gcd(\frac{\eta_{1}^{p_{1}-1}-1}{p_{1}}, p_{1}) = 1. |
Following Lemma 2.1, \mu_{1} is a primitive root modulo p_{1}^{d} for all d\geq 1 such that \mu_{1}\equiv 1 \pmod {p_{2}} . By the symmetric argument, we can find an integer \mu_{2} satisfying that
(1) \mu_{2} is a primitive root modulo p_{2}^{d} for all d\geq 1 ; and
(2) \mu_{2}\equiv 1 \pmod {p_{1}} .
We fix such a pair of integers \mu_{1} and \mu_{2} .
Theorem 3.1. Let p_{1} and p_{2} be two different odd prime numbers not dividing q , and t_{1} and t_{2} be positive integers. Then all the distinct q -cyclotomic cosets module p_{1}^{t_{1}}p_{2}^{t_{2}} are given by
C_{\mu_{1}^{k_{1}}\mu_{2}^{k_{2}}p_{1}^{r_{1}}p_{2}^{r_{2}}} = \{\mu_{1}^{k_{1}}\mu_{2}^{k_{2}}p_{1}^{r_{1}}p_{2}^{r_{2}}, \mu_{1}^{k_{1}}\mu_{2}^{k_{2}}p_{1}^{r_{1}}p_{2}^{r_{2}}q, \cdots, \mu_{1}^{k_{1}}\mu_{2}^{k_{2}}p_{1}^{r_{1}}p_{2}^{r_{2}}q^{c_{r_{1}, r_{2}}}\} |
for 0 \leq r_{1} \leq t_{1} , 0\leq r_{2} \leq t_{2} , 0 \leq k_{1} \leq g_{1, t_{1}-r_{1}} - 1 and 0 \leq k_{2} \leq g_{2, t_{2}-r_{2}}\cdot \gcd(f_{1, t_{1}-r_{1}}, f_{2, t_{2}-r_{2}}) - 1 , where c_{r_{1}, r_{2}} = \mathrm{ord}_{p_{1}^{t_{1}-r_{1}}p_{2}^{t_{2}-r_{2}}}(q) = \mathrm{lcm}(f_{1, t_{1}-r_{1}}, f_{2, t_{2}-r_{2}}) .
Proof. First we prove that the given q -cyclotomic cosets are all distinct. If C_{\mu_{1}^{k_{1}}\mu_{2}^{k_{2}}p_{1}^{r_{1}}p_{2}^{r_{2}}} = C_{\mu_{1}^{k_{1}^{\prime}}\mu_{2}^{k_{2}^{\prime}}p_{1}^{r_{1}^{\prime}}p_{2}^{r_{2}^{\prime}}} for some 0 \leq r_{1}, r_{1}^{\prime} \leq t_{1} , 0\leq r_{2}, r_{2}^{\prime} \leq t_{2} , 0 \leq k_{1}, k_{1}^{\prime} \leq g_{1, t_{1}-r_{1}} - 1 and 0 \leq k_{2}, k_{2}^{\prime} \leq g_{2, t_{2}-r_{2}}\cdot \gcd(f_{1, t_{1}-r_{1}}, f_{2, t_{2}-r_{2}})-1 , then there exists a positive integer m such that
\begin{equation} \mu_{1}^{k_{1}^{\prime}}\mu_{2}^{k_{2}^{\prime}}p_{1}^{r_{1}^{\prime}}p_{2}^{r_{2}^{\prime}} \equiv \mu_{1}^{k_{1}}\mu_{2}^{k_{2}}p_{1}^{r_{1}}p_{2}^{r_{2}}q^{m} \pmod{p_{1}^{t_{1}}p_{2}^{t_{2}}}. \end{equation} | (3.1) |
Since \mu_{1}, \mu_{2} and q are relatively prime to p_{1}^{t_{1}}p_{2}^{t_{2}} , clearly we have r_{1} = r_{1}^{\prime} and r_{2} = r_{2}^{\prime} , and Eq (3.1) can be reduced to
\mu_{1}^{k_{1}^{\prime}}\mu_{2}^{k_{2}^{\prime}} \equiv \mu_{1}^{k_{1}}\mu_{2}^{k_{2}}q^{m} \pmod{p_{1}^{t_{1}-r_{1}}p_{2}^{t_{2}-r_{2}}}. |
Remembering that \mu_{1} \equiv 1 \pmod{p_{2}} and \mu_{2} \equiv 1 \pmod{p_{1}} , then by the Chinese remainder theorem, we have
\begin{equation} \mu_{1}^{k_{1}-k_{1}^{\prime}} \equiv q^{m} \pmod{p_{1}^{t_{1}-r_{1}}} \end{equation} | (3.2) |
\begin{equation} \mu_{2}^{k_{2}-k_{2}^{\prime}} \equiv q^{m} \pmod{p_{2}^{t_{2}-r_{2}}} \end{equation} | (3.3) |
Equation (3.2) implies that
\mu_{1}^{(k_{1}-k_{1}^{\prime})f_{1, t_{1}-r_{1}}} \equiv q^{m\cdot f_{1, t_{1}-r_{1}}} \equiv 1 \pmod{p_{1}^{t_{1}-r_{1}}}, |
and therefore \phi(p_{1}^{t_{1}-r_{1}}) \mid (k_{1}-k_{1}^{\prime})f_{1, t_{1}-r_{1}} . Since 0 \leq k_{1}, k_{1}^{\prime} \leq g_{1, t_{1}-r_{1}} - 1 , one must have k_{1} = k_{1}^{\prime} . Notice that k_{1} = k_{1}^{\prime} indicates that q^{m} \equiv 1 \pmod{p_{1}^{t_{1}-r_{1}}} , then f_{1, t_{1}-r_{1}} \mid m , which together with Eq (3.3) leads to
\mu_{2}^{(k_{2}^{\prime}-k_{2})\cdot \dfrac{f_{2, t_{2}-r_{2}}}{\gcd(f_{1, t_{1}-r_{1}}, f_{2, t_{2}-r_{2}})}} \equiv q^{m \cdot \dfrac{f_{2, t_{2}-r_{2}}}{\gcd(f_{1, t_{1}-r_{1}}, f_{2, t_{2}-r_{2}})}} \equiv 1 \pmod{p_{2}^{t_{2}-r_{2}}}. |
Thus \phi(p_{2}^{t_{2}-r_{2}}) \mid (k_{2}^{\prime}-k_{2})\cdot \dfrac{f_{2, t_{2}-r_{2}}}{\gcd(f_{1, t_{1}-r_{1}}, f_{2, t_{2}-r_{2}})} . Since 0 \leq k_{2}, k_{2}^{\prime} \leq g_{2, t_{2}-r_{2}}\cdot \gcd(f_{1, t_{1}-r_{1}}, f_{2, t_{2}-r_{2}}) - 1 , we have k_{2} = k_{2}^{\prime} .
On the other hand, there are in total
\begin{equation} \begin{aligned} &\sum\limits_{0 \leq r_{1} \leq t_{1}} \sum\limits_{0 \leq r_{2} \leq t_{2}} \dfrac{\phi(p_{1}^{t_{1}-r_{1}})}{f_{1, t_{1}-r_{1}}}\cdot \dfrac{\phi(p_{2}^{t_{2}-r_{2}})}{f_{2, t_{2}-r_{2}}} \cdot \gcd(f_{1, t_{1}-r_{1}}, f_{2, t_{2}-r_{2}})\cdot \mathrm{lcm}(f_{1, t_{1}-r_{1}}, f_{2, t_{2}-r_{2}}) \\ & = \sum\limits_{0 \leq r_{1} \leq t_{1}} \sum\limits_{0 \leq r_{2} \leq t_{2}}\phi(p_{1}^{t_{1}-r_{1}})\phi(p_{2}^{t_{2}-r_{2}}) = p_{1}^{t_{1}}p_{2}^{t_{2}} \end{aligned} \end{equation} | (3.4) |
elements in these q -cyclotomic cosets, therefore they are all the distinct q -cyclotomic cosets module p_{1}^{t_{1}}p_{2}^{t_{2}} .
In particular, when t_{1} = 1 and t_{2} = t , the classification of the q -cyclotomic cosets modulo p_{1}p_{2}^{t} is given as follow.
Corollary 3.1. Let the notations be as above. Then all the distinct q -cyclotomic cosets modulo p_{1}p_{2}^{t} are
C_{0} = \{0\}; |
C_{\mu_{1}^{k_{1}}\mu_{2}^{k_{2}}p_{2}^{r}} = \{\mu_{1}^{k_{1}}\mu_{2}^{k_{2}}p_{2}^{r}, \mu_{1}^{k_{1}}\mu_{2}^{k_{2}}p_{2}^{r}q, \cdots, \mu_{1}^{k_{1}}\mu_{2}^{k_{2}}p_{2}^{r}q^{\mathrm{ord}_{p_{1}p_{2}^{t-r}}(q)-1}\} |
for 0 \leq r \leq t-1 , 0\leq k_{1}\leq g_{1} -1 and 0 \leq k_{2} \leq g_{2, t-r} \cdot \gcd(f_{1}, f_{2, t-r}) ;
C_{\mu_{1}^{k}p_{2}^{t}} = \{\mu_{1}^{k}p_{2}^{t}, \mu_{1}^{k}p_{2}^{t}q, \cdots, \mu_{1}^{k}p_{2}^{t}q^{f_{1}-1}\} |
for 0\leq k\leq g_{1}-1 ; and
C_{\mu_{2}^{k^{\prime}}p_{1}p_{2}^{r}} = \{\mu_{2}^{k^{\prime}}p_{1}p_{2}^{r}, \mu_{2}^{k^{\prime}}p_{1}p_{2}^{r}q, \cdots, \mu_{2}^{k^{\prime}}p_{1}p_{2}^{r}q^{f_{2, t-r}-1}\} |
for 0\leq r \leq t-1 and 0\leq k^{\prime} \leq g_{2, t-r}-1 .
Corollary 3.2. Let the notations be as aboved. Then the irreducible factorization of x^{p_{1}p_{2}^{t}p^{s}}-1 over \mathbb{F}_q is given by
x^{p_{1}p_{2}^{t}p^{s}}-1 = C_{0}(x)^{p^{s}}\prod\limits_{r = 0}^{t-1}\prod\limits_{k_{1} = 0}^{g_{1}-1}\prod\limits_{k_{2} = 0}^{g_{2, t-r}\gcd(f_{1}, f_{2, t-r})-1} C_{\mu_{1}^{k_{1}}\mu_{2}^{k_{2}}p_{2}^{r}}(x)^{p^{s}}\prod\limits_{k = 0}^{g_{1}-1} C_{\mu_{1}^{k}p_{2}^{t}}(x)^{p^{s}}\prod\limits_{r = 0}^{t-1}\prod\limits_{k^{\prime} = 0}^{g_{2, t-r}-1} C_{\mu_{2}^{k^{\prime}}p_{1}p_{2}^{r}}(x)^{p^{s}}. |
In this section, we will determine the generator polynomials of all constacyclic codes of length p_{1}p_{2}^{t}p^{s} over \mathbb{F}_q and their dual codes. For \lambda \in \mathbb{F}_{q}^{*} , we identify a \lambda -constacyclic code of length p_{1}p_{2}^{t}p^{s} with an ideal (g(x)) of the quotient ring \mathbb{F}_q[x]/(x^{p_{1}p_{2}^{t}p^{s}}-\lambda) , where g(x) is a divisor of x^{p_{1}p_{2}^{t}p^{s}}-\lambda . By Lemma 2.3, there are \gcd(p_{1}p_{2}^{t}, q-1) p_{1}p_{2}^{t}p^{s} -equivalence classes in \mathbb{F}_{q}^{*} , which corresponds to the cosets of \langle \xi^{p_{1}p_{2}^{t} } \rangle in \mathbb{F}_{q}^{*} = \langle \xi \rangle .
Before doing the explicit computation, we present a general method to factorize x^{n} - \lambda . Let q = p^{k} for k > 0 , and n = p^{e}p_{1}^{e_{1}} \cdots p_{m}^{e_{m}} be the prime factorization of n . Assume that p_{1}^{e_{1}} \cdots p_{m}^{e_{m}} \mid q-1 , i.e., v_{p_{i}}(q-1) \geq e_{i} for i = 1, \cdots, m . In this case we have
\mathbb{F}_{q}^* = \langle\xi\rangle = \langle\xi^{p_{1}^{e_{1}}\cdots p_{m}^{e_{m}}}\rangle \cup \langle\xi^{p_{1}^{e_{1}}\cdots p_{m}^{e_{m}}}\rangle\xi^{p^{e}} \cup \cdots\cup\langle \xi^{p_{1}^{e_{1}}\cdots p_{m}^{e_{m}}}\rangle\xi^{p^{e}(p_{1}^{e_{1}}\cdots p_{m}^{e_{m}}-1)}. |
For \lambda\in \langle\xi^{p_{1}^{e_{1}}\cdots p_{m}^{e_{m}}}\rangle\xi^{j\cdot p^{e}} , where 0\leq j\leq p_{1}^{e_{1}}\cdots p_{m}^{e_{m}}-1 , there exists an element a\in \mathbb{F}_{q}^* such that
a^{n}\lambda = \xi^{j\cdot p^{e}}. |
We first factorize x^{n} - \xi^{jp^{e}} , 0 \leq j \leq p_{1}^{e_{1}}\cdots p_{m}^{e_{m}}-1 . Notice that j can be written as j = y\cdot p_{1}^{v_{1}}\cdots p_{m}^{v_{m}} , where v_{i} = min\{e_{i}, v_{p_{i}}(j)\} . Then we have
x^{n}-\xi^{j\cdot p^{e}} = (x^{p_{1}^{e_{1}}\cdots p_{m}^{e_{m}}}-\xi^{y\cdot p_{1}^{v_{1}}\cdots p_{m}^{v_{m}}})^{p^{e}} = \xi^{j\cdot p^{e}}((\dfrac{x^{p_{1}^{e_{1}-v_{1}}\cdots p_{m}^{e_{m}-v_{m}}}}{\xi^{y}})^{p_{1}^{v_{1}}\cdots p_{m}^{v_{m}}}-1)^{p^{e}}. |
Since p_{1}^{v_{1}}\cdots p_{m}^{v_{m}} \mid q-1 , \delta = \xi^{\frac{q-1}{p_{1}^{v_{1}}\cdots p_{m}^{v_{m}}}} is a primitive p_{1}^{v_{1}}\cdots p_{m}^{v_{m}} -th root of unit. Then
\begin{equation*} \begin{array}{rcl} x^{n}-\xi^{j\cdot p^{e}}& = &\xi^{j\cdot p^{e}}(\frac{x^{p_{1}^{e_{1}-v_{1}}\cdots p_{m}^{e_{m}-v_{m}}}}{\xi^{y}}-1)^{p^{e}} \cdot(\frac{x^{p_{1}^{e_{1}-v_{1}}\cdots p_{m}^{e_{m}-v_{m}}}}{\xi^{y}}-\delta)^{p^{e}} \cdots(\frac{x^{p_{1}^{e_{1}-v_{1}}\cdots p_{m}^{e_{m}-v_{m}}}}{\xi^{y}}-\delta^{p_{1}^{v_{1}}\cdots p_{m}^{v_{m}}-1})^{p^{e}}\\ & = &(x^{p_{1}^{e_{1}-v_{1}}\cdots p_{m}^{e_{m}-v_{m}}}-\xi^{y})^{p^{e}}(x^{p_{1}^{e_{1}-v_{1}}\cdots p_{m}^{e_{m}-v_{m}}}-\delta\xi^{y})^{p^{e}} \cdots(x^{p_{1}^{e_{1}-v_{1}}\cdots p_{m}^{e_{m}-v_{m}}}-\delta^{p_{1}^{v_{1}}\cdots p_{m}^{v_{m}}-1}\xi^{y})^{p^{e}}. \end{array} \end{equation*} |
For 0\leq i\leq p_{1}^{v_{1}}\cdots p_{m}^{v_{m}}-1 , \delta^{i}\xi^{y} = \xi^{y+i\cdot \frac{q-1}{p_{1}^{v_{1}}\cdots p_{m}^{v_{m}}}} , and then we have
\mathrm{ord}(\delta^{i}\xi^{y}) = \frac{q-1}{gcd(q-1, y+i\cdot \frac{q-1}{p_{1}^{v_{1}}\cdots p_{m}^{v_{m}}})}, |
and
\frac{q-1}{\mathrm{ord}(\delta^{i}\xi^{y})} = \gcd(q-1, y+i\cdot \frac{q-1}{p_{1}^{v_{1}}\cdots p_{m}^{v_{m}}}). |
For each p_{i}\mid p_{1}^{e_{1}-v_{1}}\cdots p_{m}^{e_{m}-v_{m}} , we have that e_{i} > v_{i} and v_{i} = v_{p_{i}}(j) , thus p_{i}\nmid y . Since v_{p_{i}}(q-1)\geq e_{i} > v_{i} , p_{i}\mid \frac{q-1}{p_{1}^{v_{1}}\cdots p_{m}^{v_{m}}} , which indicates that p_{i}\nmid y+i\cdot \frac{q-1}{p_{1}^{v_{1}}\cdots p_{m}^{v_{m}}} and p_{i}\mid \frac{q-1}{y+i\cdot \frac{q-1}{p_{1}^{v_{1}}\cdots p_{m}^{v_{m}}}} . Moreover if 4\mid p_{1}^{e_{1}-v_{1}}\cdots p_{m}^{e_{m}-v_{m}} , then 4\mid p_{1}^{e_{1}}\cdots p_{m}^{e_{m}}\mid q-1 . Hence by Lemma 2.2 each x^{p_{1}^{e_{1}-v_{1}}\cdots p_{m}^{e_{m}-v_{m}}}-\xi^{y}\delta^{i} is irreducible over \mathbb{F}_q .
Notice that a^{n}\lambda = \xi^{jp^{e}} , then the irreducible factorization of x^{n} - \lambda follows immediately:
\begin{eqnarray*} x^{n}-\lambda& = &(x^{p_{1}^{e_{1}-v_{1}}\cdots p_{m}^{e_{m}-v_{m}}}-a^{-p_{1}^{e_{1}-v_{1}}\cdots p_{m}^{e_{m}-v_{m}}}\xi^{y})^{p^{e}}(x^{p_{1}^{e_{1}-v_{1}}\cdots p_{m}^{e_{m}-v_{m}}}-a^{-p_{1}^{e_{1}-v_{1}}\cdots p_{m}^{e_{m}-v_{m}}}\delta\xi^{y})^{p^{e}}\cdot\\ &&\cdot\cdots(x^{p_{1}^{e_{1}-v_{1}}\cdots p_{m}^{e_{m}-v_{m}}}-a^{-p_{1}^{e_{1}-v_{1}}\cdots p_{m}^{e_{m}-v_{m}}}\delta^{p_{1}^{v_{1}}\cdots p_{m}^{v_{m}}-1}\xi^{y})^{p^{e}}, \end{eqnarray*} |
We summerize the above discussions into the following theorem.
Theorem 4.1. Let p, p_{1}, \cdots, p_{m} be distinct prime numbers. Let q = p^{k} and n = p^{e}p_{1}^{e_{1}}\cdots p_{m}^{e_{m}} , where k, e, e_{1}, \cdots, e_{m} are positive integers. Suppose that for 1\leq i \leq m , v_{p_{i}}(q-1) \geq e_{i} . Then for any \lambda \in \mathbb{F}_{q}^{*} , there exists an element a \in \mathbb{F}_{q}^{*} such that a^{n}\lambda = \xi^{jp^{e}} , 0 \leq j \leq p_{1}^{e_{1}}\cdots p_{m}^{e_{m}} . Furthermore, writing j in the form j = y\cdot p_{1}^{v_{1}}\cdots p_{m}^{v_{m}} , where v_{i} = min\{e_{i}, v_{p_{i}}(j)\} , then
\begin{eqnarray*} x^{n}-\lambda& = &(x^{p_{1}^{e_{1}-v_{1}}\cdots p_{m}^{e_{m}-v_{m}}}-a^{-p_{1}^{e_{1}-v_{1}}\cdots p_{m}^{e_{m}-v_{m}}}\xi^{y})^{p^{e}}(x^{p_{1}^{e_{1}-v_{1}}\cdots p_{m}^{e_{m}-v_{m}}}-a^{-p_{1}^{e_{1}-v_{1}}\cdots p_{m}^{e_{m}-v_{m}}}\delta\xi^{y})^{p^{e}}\cdot\\ &&\cdot\cdots(x^{p_{1}^{e_{1}-v_{1}}\cdots p_{m}^{e_{m}-v_{m}}}-a^{-p_{1}^{e_{1}-v_{1}}\cdots p_{m}^{e_{m}-v_{m}}}\delta^{p_{1}^{v_{1}}\cdots p_{m}^{v_{m}}-1}\xi^{y})^{p^{e}}, \end{eqnarray*} |
gives the irreducible factorization of x^{n}-\lambda over \mathbb{F}_q .
Now we turn to the case that p_{1}^{e_{1}}\cdots p_{m}^{e_{m}}\nmid q-1 . Sinve \gcd(p_{1}^{e_{1}}\cdots p_{m}^{e_{m}}, q) = 1 , thus there exists a least positive integer d such that p_{1}^{e_{1}}\cdots p_{m}^{e_{m}}\mid q^{d}-1 . By the lifting-the-exponent lemma, if d' is the least positive integer such that p_{1}\cdots p_{m}\mid q^{d'}-1 , then d = d'p_{1}^{v_{1}}\cdots p_{m}^{v_{m}} , where v_{i} = \max\{e_{i}-v_{p_{i}}(q^{d'}-1), 0\} .
Let \lambda be a nonzero element in \mathbb{F}_{q} . To obtain the irreducible factorization of x^{n}-\lambda over \mathbb{F}_q , we first consider the factorization over \mathbb{F}_{q^{d}} . By Theorem 4.1, there exists a \in \mathbb{F}_{q^{d}} such that a^{n}\lambda = \zeta^{jp^{e}} , 0 \leq j \leq p_{1}^{e_{1}}\cdots p_{m}^{e_{m}} -1 . Writing j as j = y\cdot p_{1}^{v_{1}}\cdots p_{m}^{v_{m}} , where v_{i} = min\{e_{i}, v_{p_{i}}(j)\} , then
\begin{eqnarray*} x^{n}-\lambda& = &(x^{p_{1}^{e_{1}-v_{1}}\cdots p_{m}^{e_{m}-v_{m}}}-a^{-p_{1}^{e_{1}-v_{1}}\cdots p_{m}^{e_{m}-v_{m}}}\zeta^{y})^{p^{e}}(x^{p_{1}^{e_{1}-v_{1}}\cdots p_{m}^{e_{m}-v_{m}}}-a^{-p_{1}^{e_{1}-v_{1}}\cdots p_{m}^{e_{m}-v_{m}}}\delta\zeta^{y})^{p^{e}}\cdot\\ &&\cdot\cdots(x^{p_{1}^{e_{1}-v_{1}}\cdots p_{m}^{e_{m}-v_{m}}}-a^{-p_{1}^{e_{1}-v_{1}}\cdots p_{m}^{e_{m}-v_{m}}}\delta^{p_{1}^{v_{1}}\cdots p_{m}^{v_{m}}-1}\zeta^{y})^{p^{e}}, \end{eqnarray*} |
gives the irreducible factorization of x^{n}-\lambda over \mathbb{F}_{q^{d}} , where \delta is a primitive p_{1}^{v_{1}}\cdots p_{m}^{v_{m}} -th root of unit. Hence each irreducible factor of x^{n}-\lambda over \mathbb{F}_q is of the form
\begin{eqnarray*} &&(x^{p_{1}^{e_{1}-v_{1}}\cdots p_{m}^{e_{m}-v_{m}}}-a^{-p_{1}^{e_{1}-v_{1}}\cdots p_{m}^{e_{m}-v_{m}}}\delta^{i}\zeta^{y})^{p^{e}}(x^{p_{1}^{e_{1}-v_{1}}\cdots p_{m}^{e_{m}-v_{m}}}-a^{-qp_{1}^{e_{1}-v_{1}}\cdots p_{m}^{e_{m}-v_{m}}}\delta^{qi}\zeta^{qy})^{p^{e}}\cdot\\ &&\cdot\cdots(x^{p_{1}^{e_{1}-v_{1}}\cdots p_{m}^{e_{m}-v_{m}}}-a^{-q^{z_{i}-1}p_{1}^{e_{1}-v_{1}}\cdots p_{m}^{e_{m}-v_{m}}}\delta^{i\cdot q^{z_{i}-1}}\zeta^{y\cdot q^{z_{i}-1}})^{p^{e}}, \end{eqnarray*} |
where z_{i} is the least positive integer such that a^{-q^{z_{i}}p_{1}^{e_{1}-v_{1}}\cdots p_{m}^{e_{m}-v_{m}}}\delta^{i\cdot q^{z_{i}}}\zeta^{y\cdot q^{z_{i}}} = a^{-p_{1}^{e_{1}-v_{1}}\cdots p_{m}^{e_{m}-v_{m}}}\delta^{i}\zeta^{y} .
Now we determine the generator polynomials of all constacyclic codes of length p_{1}p_{2}^{t}p^{s} and their duals explicitly. We decompose the problem into three cases.
As \gcd(q-1, p_{1}p_{2}^{t}p^{s}) = 1 , all constacyclic codes of length p_{1}p_{2}^{t}p^{s} are equivalent to a cyclic code. By the factorization of x^{p_{1}p_{2}^{t}p^{s}}-1 given in Corollary 3.2, we have the following result. For any polynomial
F = a_{0}+ a_{1}x +\cdots + a_{n}x^{n}, \ a_{n} \neq 0, |
we set \widehat{F} = a_{n}^{-1}F to be the monic polynomial associated to F .
Proposition 4.1. Assume that \gcd(q-1, p_{1}p_{2}^{t}p^{s}) = 1 . Then any nonzero element \lambda in \mathbb{F}_{q} is p_{1}p_{2}^{t}p^{s} -equivalent to 1, that is, there is an element a \in \mathbb{F}_{q}^{*} such that a^{p_{1}p_{2}^{t}p^{s}}\lambda = 1 . Furthermore, the irreducible factorization of x^{p_{1}p_{2}^{t}p^{s}}-\lambda over \mathbb{F}_q is given by
x^{p_{1}p_{2}^{t}p^{s}}-\lambda = \widehat{C}_{0}(ax)^{p^{s}}\prod\limits_{r = 0}^{t-1}\prod\limits_{k_{1} = 0}^{g_{1}-1}\prod\limits_{k_{2} = 0}^{g_{2, t-r}\gcd(f_{1}, f_{2, t-r})-1} \widehat{C}_{\mu_{1}^{k_{1}}\mu_{2}^{k_{2}}p_{2}^{r}}(ax)^{p^{s}}\prod\limits_{k = 0}^{g_{1}-1} \widehat{C}_{\mu_{1}^{k}p_{2}^{t}}(ax)^{p^{s}}\prod\limits_{r = 0}^{t-1}\prod\limits_{k^{\prime} = 0}^{g_{2, t-r}-1} \widehat{C}_{\mu_{2}^{k^{\prime}}p_{1}p_{2}^{r}}(ax)^{p^{s}}. |
Therefore all the constacyclic codes of length p_{1}p_{2}^{t}p^{s} are
C = \left(\widehat{C}_{0}(ax)^{u}\prod\limits_{r = 0}^{t-1}\prod\limits_{k_{1} = 0}^{g_{1}-1}\prod\limits_{k_{2} = 0}^{g_{2, t-r}\gcd(f_{1}, f_{2, t-r})-1} \widehat{C}_{\mu_{1}^{k_{1}}\mu_{2}^{k_{2}}p_{2}^{r}}(ax)^{v_{\mu_{1}^{k_{1}}\mu_{2}^{k_{2}}p_{2}^{r}}}\prod\limits_{k = 0}^{g_{1}-1} \widehat{C}_{\mu_{1}^{k}p_{2}^{t}}(ax)^{w_{\mu_{1}^{k}p_{2}^{t}}}\prod\limits_{r = 0}^{t-1}\prod\limits_{k^{\prime} = 0}^{g_{2, t-r}-1} \widehat{C}_{\mu_{2}^{k^{\prime}}p_{1}p_{2}^{r}}(ax)^{x_{\mu_{2}^{k^{\prime}}p_{1}p_{2}^{r}}}\right), |
where 0\leq u, v_{\mu_{1}^{k_{1}}\mu_{2}^{k_{2}}p_{2}^{r}}, w_{\mu_{1}^{k}p_{2}^{t}}, x_{\mu_{2}^{k^{\prime}}p_{1}p_{2}^{r}}\leq p^{s} , with duals
\begin{eqnarray*} C^{\bot}& = &\left(\widehat{C}_{0}(a^{-1}x)^{p^{s}-u}\prod\limits_{r = 0}^{t-1}\prod\limits_{k_{1} = 0}^{g_{1}-1}\prod\limits_{k_{2} = 0}^{g_{2, t-r}\gcd(f_{1}, f_{2, t-r})-1} \widehat{C}_{\mu_{1}^{k_{1}}\mu_{2}^{k_{2}}p_{2}^{r}}(a^{-1}x)^{p^{s}-v_{\mu_{1}^{k_{1}}\mu_{2}^{k_{2}}p_{2}^{r}}}\prod\limits_{k = 0}^{g_{1}-1} \widehat{C}_{\mu_{1}^{k}p_{2}^{t}}(a^{-1}x)^{p^{s}-w_{\mu_{1}^{k}p_{2}^{t}}}\right.\\ &&\left.\prod\limits_{r = 0}^{t-1}\prod\limits_{k^{\prime} = 0}^{g_{2, t-r}-1} \widehat{C}_{\mu_{2}^{k^{\prime}}p_{1}p_{2}^{r}}(a^{-1}x)^{p^{s}-x_{\mu_{2}^{k^{\prime}}p_{1}p_{2}^{r}}}\right). \end{eqnarray*} |
For this case, since p_{1}p_{2}^{t}\vert q-1 , the following proposition follows straightly from Theorem 4.1.
Theorem 4.2. Assume that \gcd(q-1, p_{1}p_{2}^{t}p^{s}) = p_{1}p_{2}^{t} . Then for any \lambda\in \mathbb{F}_{q}^* , there exists an element a\in \mathbb{F}_{q}^* such that a^{p_{1}p_{2}^{t}p^{s}}\lambda = \xi^{j\cdot p^{s}} , 0\leq j\leq p_{1}p_{2}^{t}-1 . Writing j as j = y\cdot p_{1}^{v_{1}}p_{2}^{v_{2}} , where v_{1} = \min\{1, v_{p_{1}}(j)\} and v_{2} = \min\{t, v_{p_{2}}(j)\} , then
\begin{eqnarray*} x^{p_{1}p_{2}^{t}p^{s}}-\lambda& = &(x^{p_{1}^{1-v_{1}}p_{2}^{t-v_{2}}}-a^{-p_{1}^{1-v_{1}}p_{2}^{t-v_{2}}}\xi^{y})^{p^{s}}(x^{p_{1}^{1-v_{1}}p_{2}^{t-v_{2}}}-a^{-p_{1}^{1-v_{1}}p_{2}^{t-v_{2}}}\delta\xi^{y})^{p^{s}}\\ &&\cdots(x^{p_{1}^{1-v_{1}}p_{2}^{t-v_{2}}}-a^{-p_{1}^{1-v_{1}}p_{2}^{t-v_{2}}}\delta^{p_{1}^{v_{1}}p_{2}^{v_{2}}-1}\xi^{y})^{p^{s}} \end{eqnarray*} |
gives the irreducible factorization of x^{p_{1}p_{2}^{t}p^{s}}-\lambda over \mathbb{F}_{q} . Therefore all the \lambda -constacyclic codes of length p_{1}p_{2}^{t}p^{s} and their dual codes are given by
\begin{eqnarray*} C& = &\left((x^{p_{1}^{1-v_{1}}p_{2}^{t-v_{2}}}-a^{-p_{1}^{1-v_{1}}p_{2}^{t-v_{2}}}\xi^{y})^{u_{1}}(x^{p_{1}^{1-v_{1}}p_{2}^{t-v_{2}}}-a^{-p_{1}^{1-v_{1}}p_{2}^{t-v_{2}}}\delta\xi^{y})^{u_{2}}\right.\\ &&\left.\cdots(x^{p_{1}^{1-v_{1}}p_{2}^{t-v_{2}}}-a^{-p_{1}^{1-v_{1}}p_{2}^{t-v_{2}}}\delta^{p_{1}^{v_{1}}p_{2}^{v_{2}}-1}\xi^{y})^{u _{p_{1}^{v_{1}}p_{2}^{v_{2}}}}\right), \end{eqnarray*} |
and
\begin{eqnarray*} C^{\bot}& = &\left((x^{p_{1}^{1-v_{1}}p_{2}^{t-v_{2}}}-a^{p_{1}^{1-v_{1}}p_{2}^{t-v_{2}}}\xi^{-y})^{p^{s}-u_{1}}(x^{p_{1}^{1-v_{1}}p_{2}^{t-v_{2}}}-a^{p_{1}^{1-v_{1}}p_{2}^{t-v_{2}}}\delta^{-1}\xi^{-y})^{p^{s}-u_{2}}\right.\\ &&\left.\cdots(x^{p_{1}^{1-v_{1}}p_{2}^{t-v_{2}}}-a^{p_{1}^{1-v_{1}}p_{2}^{t-v_{2}}}\delta^{1-p_{1}^{v_{1}}p_{2}^{v_{2}}}\xi^{-y})^{p^{s}-u_{p_{1}^{v_{1}}p_{2}^{v_{2}}}}\right), \end{eqnarray*} |
where 0\leq u_{1}, u_{2}, \cdots, u_{n^{v_{1}}\ell^{v_{2}}}\leq p^{s} .
In this case, for any d\geq 1 we have f_{2, d} = p_{2}^{max\{0, d-r\}} , and f = \mathrm{lcm}(f_{1}, f_{2, t}) is the least positive integer such that q^{f}\equiv 1 \pmod {p_1p_{2}^{t}} . By the bais results of finite fields, there is a primitive element \zeta in \mathbb{F}_{q^{f}}^{*} such that \xi = \zeta^{\frac{q^{f}-1}{q-1}} = \zeta^{1+q+\cdots+q^{f-1}} . Then we have
\mathbb{F}_{q}^* = \langle\xi\rangle = \langle\xi^{p_{2}^{r}}\rangle\cup \langle\xi^{p_{2}^{r}}\rangle\xi^{p^{s}}\cup \cdots\cup \langle\xi^{p_{2}^{r}}\rangle\xi^{(p_{2}^{r}-1)p^{s}} |
and
\mathbb{F}_{q^{f}}^* = \langle\zeta\rangle = \langle\zeta^{p_{1}p_{2}^{t}}\rangle\cup \langle\zeta^{p_{1}p_{2}^{t}}\rangle\zeta^{p^{s}}\cup \cdots\cup \langle\zeta^{p_{1}p_{2}^{t}}\rangle\zeta^{(p_{1}p_{2}^{t} -1)p^{s}}. |
By the assumption that p_{1}p_{2}^{t}\mid q^{f}-1 and v_{p_{1}}(q-1) = 0 , v_{p_{2}}(q-1) = r , we have that p_{1}p_{2}^{t-r}\mid (1+q+\cdots+q^{f-1}) . Therefore \xi^{p_{2}^{r}} = \zeta^{p_{2}^{r}(1+q+\cdots+q^{f-1})}\in \langle\zeta^{p_{1}p_{2}^{t}}\rangle . Furthermore, for 0\leq j\leq p_{2}^{r}-1 , there exists some 0\leq j^{'}\leq p_{1}p_{2}^{t}-1 such that jp^{s}(1+q+\cdots+q^{f-1})\equiv j^{'}p^{s} \pmod {p_{1}p_{2}^{t}} , that is, \xi^{jp^{s}}\in \langle\zeta^{p_{1}p_{2}^{t}}\rangle\zeta^{j^{'}p^{s}} . Hence we have the following theorem.
Theorem 4.3. Assume that \gcd(q-1, p_{1}p_{2}^{t}p^{s}) = p_{2}^{r} , 0 < r \leq t . For any 0\leq j\leq p_{2}^{r}-1 , there exists an element a\in \mathbb{F}_{q^{f}}^{*} such that a^{p_{1}p_{2}^{t}p^{s}}\xi^{j\cdot p^{s}} = \zeta^{j'\cdot p^{s}} . Moreover, each irreducible factor of x^{p_{1}p_{2}^{t}}-\xi^{j} over \mathbb{F}_{q} is of the form
\begin{eqnarray*} &&(x^{p_{1}^{1-v_{1}}p_{2}^{t-v_{2}}}-a^{-p_{1}^{1-v_{1}}p_{2}^{t-v_{2}}}\delta^{i}\zeta^{y'})(x^{p_{1}^{1-v_{1}}p_{2}^{t-v_{2}}}-a^{-p_{1}^{1-v_{1}}p_{2}^{t-v_{2}}\cdot q}\delta^{iq}\zeta^{y'q})\\ &&\cdots(x^{p_{1}^{1-v_{1}}p_{2}^{t-v_{2}}}-a^{-p_{1}^{1-v_{1}}p_{2}^{t-v_{2}}\cdot q^{z_{i}-1}}\delta^{iq^{z_{i}-1}}\zeta^{y'q^{z_{i}-1}}), \end{eqnarray*} |
where j' = y'p_{1}^{v_{1}}p_{2}^{v_{2}} , v_{1} = \min\{1, v_{p_{1}}(j')\} , v_{2} = \min\{t, v_{p_{2}}(j')\} , and z_{i} is the least positive integer such that a^{-q^{z_{i}}p_{1}^{1-v_{1}}p_{2}^{t-v_{2}}}\delta^{iq^{z_{i}}}\zeta^{y'q^{z_{i}}} = a^{p_{1}^{1-v_{1}}p_{2}^{t-v_{2}}}\delta^{i}\zeta^{y'} .
For any 0\leq i, i^{'}\leq p_{1}^{v_{1}}p_{2}^{v_{2}}-1 , we define a relation \sim to be such that i\sim i^{'} if and only if a^{-q^{m}p_{1}^{1-v_{1}}p_{2}^{t-v_{2}}}\delta^{iq^{m}}\zeta^{y'q^{m}} = a^{p_{1}^{1-v_{1}}p_{2}^{t-v_{2}}}\delta^{i^{'}}\zeta^{y'} for some nonnegative integers m . It is obvious to see that \sim is an equivalence relation. Assume that S is a complete system of equivalence class representatives of \{0, 1, \cdots, p_{1}^{v_{1}}p_{•}^{v_{2}}-1\} relative to this relation \sim . For any i\in S we denote the irreducible polynomial
\begin{eqnarray*} &&(x^{p_{1}^{1-v_{1}}p_{2}^{t-v_{2}}}-a^{-p_{1}^{1-v_{1}}p_{2}^{t-v_{2}}}\delta^{i}\zeta^{y'})(x^{p_{1}^{1-v_{1}}p_{2}^{t-v_{2}}}-a^{-p_{1}^{1-v_{1}}p_{2}^{t-v_{2}}\cdot q}\delta^{iq}\zeta^{y'q})\\ &&\cdots(x^{p_{1}^{1-v_{1}}p_{2}^{t-v_{2}}}-a^{-p_{1}^{1-v_{1}}p_{2}^{t-v_{2}}\cdot q^{z_{i}-1}}\delta^{iq^{z_{i}-1}}\zeta^{y'q^{z_{i}-1}}), \end{eqnarray*} |
by M_{i}(x) . Then we have the following corollary.
Corollary 4.1. Assume that \gcd(q-1, p_{1}p_{2}^{t}p^{s}) = p_{2}^{r} . For any 0\leq j\leq p_{2}^{r}-1 , there exists an element a\in \mathbb{F}_{q^{f}}^{*} such that a^{p_{1}p_{2}^{t}p^{s}}\xi^{j\cdot p^{s}} = \zeta^{j'\cdot p^{s}} . Then
x^{p_{1}p_{2}^{t}p^{s}}-\xi^{j p^{s}} = \prod\limits_{i\in S}M_{i}(x)^{p^{s}} |
gives the irreducible factorization of x^{p_{1}p_{2}^{t}p^{s}}-\xi^{j p^{s}} over \mathbb{F}_q . Furthermore we have that
C = \left(\prod\limits_{i\in S}M_{i}(x)^{u_{i}}\right), |
and
C^{\bot} = \left(\prod\limits_{i\in S}M_{i}^{*}(x)^{p^{s}-u_{i}}\right), |
where 0\leq u_{i}\leq p^{s} , for i\in S .
The same argument as in the last section can be applied in this situation, only noticing that the least positive integer f such that q^{f}\equiv 1 \pmod {p_1p_2^t} is f = f_{2, t} = p_2^{\max\{0, t-r\}} . We find a primitive element \zeta in \mathbb{F}_{q^{f}}^{*} such that \xi = \zeta^{\frac{q^{f}-1}{q-1}} = \zeta^{1+q+\cdots+q^{f-1}} , then
\mathbb{F}_{q}^* = \langle\xi\rangle = \langle\xi^{p_{1}p_{2}^{r}}\rangle\cup \langle\xi^{p_{1}p_{2}^{r}}\rangle\xi^{p^{s}}\cup \cdots\cup \langle\xi^{p_{1}p_{2}^{r}}\rangle\xi^{(p_{2}^{r}-1)p^{s}} |
and
\mathbb{F}_{q^{f}}^* = \langle\zeta\rangle = \langle\zeta^{p_{1}p_{2}^{t}}\rangle\cup \langle\zeta^{p_{1}p_{2}^{t}}\rangle\zeta^{p^{s}}\cup \cdots\cup \langle\zeta^{p_{1}p_{2}^{t}}\rangle\zeta^{(p_{1}p_{2}^{t} -1)p^{s}}. |
By the assumption that p_{1}p_{2}^{t}\mid q^{f}-1 and v_{p_{2}}(q-1) = r , we have that p_{2}^{t-r}\mid (1+q+\cdots+q^{f-1}) , and \xi^{p_{1}p_{2}^{r}} = \zeta^{p_{1}p_{2}^{r}(1+q+\cdots+q^{f-1})}\in \langle\zeta^{p_{1}p_{2}^{t}}\rangle . Furthermore, for 0\leq j\leq p_{1}p_{2}^{r}-1 , there exists some 0\leq j^{'}\leq p_{1}p_{2}^{t}-1 such that jp^{s}(1+q+\cdots+q^{f-1})\equiv j^{'}p^{s} \pmod {p_1p_2^t} , that is, \xi^{jp^{s}}\in \langle\zeta^{p_{1}p_{2}^{t}}\rangle\zeta^{j^{'}p^{s}} . Hence we have the following theorem.
Theorem 4.4. Assume that \gcd(q-1, p_{1}p_{2}^{t}p^{s}) = p_{1}p_{2}^{r} for 0 < r < t , then for any 0\leq j\leq p_{2}^{r}-1 , there exists an element a\in \mathbb{F}_{q^{f}}^{*} such that a^{p_{1}p_{2}^{t}p^{s}}\xi^{j\cdot p^{s}} = \zeta^{j'\cdot p^{s}} . Moreover, each irreducible factor of x^{p_{1}p_{2}^{t}}-\xi^{j} over \mathbb{F}_{q} is of the form
\begin{eqnarray*} &&(x^{p_{1}^{1-v_{1}}p_{2}^{t-v_{2}}}-a^{-p_{1}^{1-v_{1}}p_{2}^{t-v_{2}}}\delta^{i}\zeta^{y'})(x^{p_{1}^{1-v_{1}}p_{2}^{t-v_{2}}}-a^{-p_{1}^{1-v_{1}}p_{2}^{t-v_{2}}\cdot q}\delta^{iq}\zeta^{y'q})\\ &&\cdots(x^{p_{1}^{1-v_{1}}p_{2}^{t-v_{2}}}-a^{-p_{1}^{1-v_{1}}p_{2}^{t-v_{2}}\cdot q^{z_{i}-1}}\delta^{iq^{z_{i}-1}}\zeta^{y'q^{z_{i}-1}}), \end{eqnarray*} |
where j' = y'p_{1}^{v_{1}}p_{2}^{v_{2}} , v_{1} = \min\{1, v_{p_{1}}(j')\} , v_{2} = \min\{t, v_{p_{2}}(j')\} , and z_{i} is the least positive integer such that a^{-q^{z_{i}}p_{1}^{1-v_{1}}p_{2}^{t-v_{2}}}\delta^{iq^{z_{i}}}\zeta^{y'q^{z_{i}}} = a^{p_{1}^{1-v_{1}}p_{2}^{t-v_{2}}}\delta^{i}\zeta^{y'} .
For any 0\leq i, i^{'}\leq p_{1}^{v_{1}}p_{2}^{v_{2}}-1 , we define a relation \sim to be such that i\sim i^{'} if and only if a^{-q^{m}p_{1}^{1-v_{1}}p_{2}^{t-v_{2}}}\delta^{iq^{m}}\zeta^{y'q^{m}} = a^{p_{1}^{1-v_{1}}p_{2}^{t-v_{2}}}\delta^{i^{'}}\zeta^{y'} for some nonnegative integers m . It is obvious to see that \sim is an equivalence relation. Assume that S is a complete system of equivalence class representatives of \{0, 1, \cdots, p_{1}^{v_{1}}p_2^{v_{2}}-1\} relative to this relation \sim . For any i\in S we denote the irreducible polynomial
\begin{eqnarray*} &&(x^{p_{1}^{1-v_{1}}p_{2}^{t-v_{2}}}-a^{-p_{1}^{1-v_{1}}p_{2}^{t-v_{2}}}\delta^{i}\zeta^{y'})(x^{p_{1}^{1-v_{1}}p_{2}^{t-v_{2}}}-a^{-p_{1}^{1-v_{1}}p_{2}^{t-v_{2}}\cdot q}\delta^{iq}\zeta^{y'q})\\ &&\cdots(x^{p_{1}^{1-v_{1}}p_{2}^{t-v_{2}}}-a^{-p_{1}^{1-v_{1}}p_{2}^{t-v_{2}}\cdot q^{z_{i}-1}}\delta^{iq^{z_{i}-1}}\zeta^{y'q^{z_{i}-1}}), \end{eqnarray*} |
by M_{i}(x) . Then we have the following corollary.
Corollary 4.2. Assume that \gcd(q-1, p_{1}p_{2}^{t}p^{s}) = p_{1}p_{2}^{r} for 0 < r < t . For any 0\leq j\leq p_{2}^{r}-1 , there exists an element a\in \mathbb{F}_{q^{f}}^{*} such that a^{p_{1}p_{2}^{t}p^{s}}\xi^{j\cdot p^{s}} = \zeta^{j'\cdot p^{s}} . Then
x^{p_{1}p_{2}^{t}p^{s}}-\xi^{j p^{s}} = \prod\limits_{i\in Z}M_{i}(x)^{p^{s}} |
gives the irreducible factorization of x^{p_{1}p_{2}^{t}p^{s}}-\xi^{j p^{s}} over \mathbb{F}_q . Furthermore we have that
C = \left(\prod\limits_{i\in S}M_{i}(x)^{u_{i}}\right), |
and
C^{\bot} = \left(\prod\limits_{i\in S}M_{i}^{*}(x)^{p^{s}-u_{i}}\right), |
where 0\leq u_{i}\leq p^{s} , for i\in S .
Based on the results in the last section, we now give all the self-dual cyclic codes of length p_{1}p_{2}^{t}p^{s} over \mathbb{F}_q and their enumeration. It is a well-known conclusion that self-dual cyclic codes of length N over \mathbb{F}_q exists if and only if N is even and the characteristic of \mathbb{F}_q is 2 . Therefore we only consider the case of self-dual cyclic codes of length p_{1}p_{2}^{t}\cdot 2^{s} over \mathbb{F}_{2^{k}} .
Let x^{p_{1}p_{2}^{t}2^{s}}-1 = (x^{p_{1}p_{2}^{t}}-1)^{2^{s}} = f_{1}(x)^{2^{s}}\cdots f_{n}(x)^{2^{s}}h_{1}(x)^{2^{s}}\cdots h_{m}(x)^{2^{s}}h_{1}^{*}(x)^{2^{s}}\cdots h_{m}^{*}(x)^{2^{s}} be the irreducible factorization of x^{p_{1}p_{2}^{t}2^{s}}-1 over \mathbb{F}_q , where each f_{i}(x) is a monic irreducible self-reciprocal polynomial for 1\leq i\leq n , and h_{j}^{*}(x) is the reciprocal polynomial of h_{j}(x) for each 1\leq j\leq m . Now, given a cyclic code C = (g(x)) of length p_{1}p_{2}^{t}2^{s} , it can be written in the form
g(x) = f_{1}(x)^{\tau_{1}}\cdots f_{n}(x)^{\tau_{n}}h_{1}(x)^{\delta_{1}}\cdots h_{m}(x)^{\delta_{m}}h_{1}^{*}(x)^{\sigma_{1}}\cdots h_{m}^{*}(x)^{\sigma_{m}}, |
where 0\leq \tau_{i}, \delta_{j}, \sigma_{j}\leq 2^{s} for any 1\leq i\leq n and 1\leq j\leq m . Then the reciprocal polynomial h^{*}(x) of the parity check polynomial h(x) of C is
h^{*}(x) = f_{1}(x)^{2^{s}-\tau_{1}}\cdots f_{n}(x)^{2^{s}-\tau_{n}}h_{1}(x)^{2^{s}-\sigma_{1}}\cdots h_{m}(x)^{2^{s}-\sigma_{m}}h_{1}^{*}(x)^{2^{s}-\delta_{1}}\cdots h_{m}^{*}(x)^{2^{s}-\delta_{m}}. |
Therefore it is obvious to see that the following theorem holds.
Theorem 5.1. With the above notations, we have that C is self-dual if and only if 2\tau_{i} = 2^{s} for 1\leq i\leq n , and \delta_{j}+\sigma_{j} = 2^{s} for 1\leq j\leq m .
Recall the irreducible factorization of x^{p_{1}p_{2}^{t}p^{s}}-1 given in Corollary 3.2. Now we determine for each irreducible factor its reciprocal polynomial.
Lemma 5.1. Let the notations be defined as Corollary 3.1. Then one of the following holds.
(1) If both f_{1} and f_{2} are odd, then we have that
C_{0}^{*} = C_{0}, \ C_{\mu_{1}^{k}p_{2}^{t}}^{*} = C_{-\mu_{1}^{k}p_{2}^{t}}, \ C_{\mu_{2}^{k^{\prime}}p_{1}p_{2}^{r}}^{*} = C_{-\mu_{2}^{k^{\prime}}p_{1}p_{2}^{r}}, \ C_{\mu_{1}^{k_{1}}\mu_{2}^{k_{2}}p_{2}^{r}}^{*} = C_{-\mu_{1}^{k_{1}}\mu_{2}^{k_{2}}p_{2}^{r}}. |
(2) If f_{1} is odd and f_{2} is even, then we have that
C_{0}^{*} = C_{0}, \ C_{\mu_{1}^{k}p_{2}^{t}}^{*} = C_{-\mu_{1}^{k}p_{2}^{t}}, \ C_{\mu_{2}^{k^{\prime}}p_{1}p_{2}^{r}}^{*} = C_{\mu_{2}^{k^{\prime}}p_{1}p_{2}^{r}}, \ C_{\mu_{1}^{k_{1}}\mu_{2}^{k_{2}}p_{2}^{r}}^{*} = C_{-\mu_{1}^{k_{1}}\mu_{2}^{k_{2}}p_{2}^{r}}. |
(3) If f_{1} is even and f_{2} is odd, then we have that
C_{0}^{*} = C_{0}, \ C_{\mu_{1}^{k}p_{2}^{t}}^{*} = C_{\mu_{1}^{k}p_{2}^{t}}, \ C_{\mu_{2}^{k^{\prime}}p_{1}p_{2}^{r}}^{*} = C_{-\mu_{2}^{k^{\prime}}p_{1}p_{2}^{r}}, \ C_{\mu_{1}^{k_{1}}\mu_{2}^{k_{2}}p_{2}^{r}}^{*} = C_{-\mu_{1}^{k_{1}}\mu_{2}^{k_{2}}p_{2}^{r}}. |
(4) If both f_{1} and f_{2} are even, then we have when v_{2}(f_{1})\neq v_{2}(f_{2}) ,
C_{0}^{*} = C_{0}, \ C_{\mu_{1}^{k}p_{2}^{t}}^{*} = C_{\mu_{1}^{k}p_{2}^{t}}, \ C_{\mu_{2}^{k^{\prime}}p_{1}p_{2}^{r}}^{*} = C_{\mu_{2}^{k^{\prime}}p_{1}p_{2}^{r}}, \ C_{\mu_{1}^{k_{1}}\mu_{2}^{k_{2}}p_{2}^{r}}^{*} = C_{-\mu_{1}^{k_{1}}\mu_{2}^{k_{2}}p_{2}^{r}}, |
when v_{2}(f_{1}) = v_{2}(f_{2}) ,
C_{0}^{*} = C_{0}, \ C_{\mu_{1}^{k}p_{2}^{t}}^{*} = C_{\mu_{1}^{k}p_{2}^{t}}, \ C_{\mu_{2}^{k^{\prime}}p_{1}p_{2}^{r}}^{*} = C_{\mu_{2}^{k^{\prime}}p_{1}p_{2}^{r}}, \ C_{\mu_{1}^{k_{1}}\mu_{2}^{k_{2}}p_{2}^{r}}^{*} = C_{-\mu_{1}^{k_{1}}\mu_{2}^{k_{2}}p_{2}^{r}}. |
Proof. First it is trivial that the reciprocal of C_{0} is always itself. For C_{\mu_{1}^{k}p_{2}^{t}} , notice that C_{\mu_{1}^{k}p_{2}^{t}}^{*} = C_{\mu_{1}^{k}p_{2}^{t}} if and only if the congruence equation -\mu_{1}^{k}p_{2}^{t}\equiv -\mu_{1}^{k}p_{2}^{t}q^{x} \pmod {p_1p_{2}^{t}} is solvable. Since the equation is equivalent to -1\equiv q^{x} \pmod {p_2^{r}} , then the condition holds if and only if f_{1} = \mathrm{ord}_{p_{1}}(q) is even. In the similar way we can check that C_{\mu_{2}^{k^{\prime}}p_{1}p_{2}^{r}}^{*} = C_{\mu_{2}^{k^{\prime}}p_{1}p_{2}^{r}} if and only if f_{2, t-r} = f_{2}p_{2}^{max\{0, t-r\}} is even. Notice that by assumption p_{2} is odd, therefore the condition holds if and only if f_{2} is even. For C_{\mu_{1}^{k_{1}}\mu_{2}^{k_{2}}p_{2}^{r}} , consider the congruence equation -\mu_{1}^{k_{1}}\mu_{2}^{k_{2}}p_{2}^{r}\equiv \mu_{1}^{k_{1}}\mu_{2}^{k_{2}}p_{2}^{r}q^{x} \pmod {p_1p_{2}^{t}} . It is equivalent to that -1\equiv q^{x} \pmod {p_1} and -1\equiv q^{x} \pmod {p_2^{t-r}} holds simultaneously. This requires not only both f_{1} and f_{2} are even, but also \gcd(f_{1}, f_{2, t-r})\mid\dfrac{f_{1}-f_{2, t-r}}{2} . And it is trivial to check that the last condition holds if and only if v_{2}(f_{1}) = v_{2}(f_{2, t-r}) = v_{2}(f_{2}) .
Based on the above lemma, we now determine all the self-dual cyclic codes of length p_{1}p_{2}^{t} and their enumeration.
Theorem 5.2.
(1) If both f_{1} and f_{2} are odd, then there exist (2^{s}+1)^{\frac{p_{1}p_{2}^{t}-1}{2}} self-dual cyclic codes of length p_{1}p_{2}^{t} over \mathbb{F}_{2^{k}} , which are given by
\begin{eqnarray*} &\left((x-1)^{2^{s-1}}\prod\nolimits_{r = 0}^{t-1}\prod\nolimits_{k_{1} = 0}^{\frac{g_{1}}{2}-1}\prod\nolimits_{k_{2} = 0}^{g_{2, t-r}\gcd(f_{1}, f_{2, t-r})-1} C_{\mu_{1}^{k_{1}}\mu_{2}^{k_{2}}p_{2}^{r}}(x)^{v_{\mu_{1}^{k_{1}}\mu_{2}^{k_{2}}p_{2}^{r}}} C_{-\mu_{1}^{k_{1}}\mu_{2}^{k_{2}}p_{2}^{r}}(x)^{2^{s}-v_{\mu_{1}^{k_{1}}\mu_{2}^{k_{2}}p_{2}^{r}}} \right.\\ &\left.\cdot\prod\nolimits_{k = 0}^{\frac{g_{1}}{2}-1} C_{\mu_{1}^{k}p_{2}^{t}}(x)^{w_{\mu_{1}^{k}p_{2}^{t}}} C_{-\mu_{1}^{k}p_{2}^{t}}(x)^{2^{s}-w_{\mu_{1}^{k}p_{2}^{t}}} \prod\nolimits_{r = 0}^{t-1}\prod\nolimits_{k^{\prime} = 0}^{\frac{g_{2, t-r}\gcd(f_{1}, f_{2, t-r})}{2}-1} C_{\mu_{2}^{k^{\prime}}p_{1}p_{2}^{r}}(x)^{x_{\mu_{2}^{k^{\prime}}p_{1}p_{2}^{r}}} C_{-\mu_{2}^{k^{\prime}}p_{1}p_{2}^{r}}(x)^{2^{s}-x_{\mu_{2}^{k^{\prime}}p_{1}p_{2}^{r}}}\right). \end{eqnarray*} |
(2) If f_{1} is odd and f_{2} is even, then there exist (2^{s}+1)^{\frac{p_{1}(p_{2}^{t}-1)}{2}} self-dual cyclic codes of length p_{1}p_{2}^{t} over \mathbb{F}_{2^{k}} , which are given by
\begin{eqnarray*} &\left((x-1)^{2^{s-1}}\prod\nolimits_{r = 0}^{t-1}\prod\nolimits_{k_{1} = 0}^{\frac{g_{1}}{2}-1}\prod\nolimits_{k_{2} = 0}^{g_{2, t-r}\gcd(f_{1}, f_{2, t-r})-1} C_{\mu_{1}^{k_{1}}\mu_{2}^{k_{2}}p_{2}^{r}}(x)^{v_{\mu_{1}^{k_{1}}\mu_{2}^{k_{2}}p_{2}^{r}}} C_{-\mu_{1}^{k_{1}}\mu_{2}^{k_{2}}p_{2}^{r}}(x)^{2^{s}-v_{\mu_{1}^{k_{1}}\mu_{2}^{k_{2}}p_{2}^{r}}} \right.\\ &\left.\cdot\prod\nolimits_{k = 0}^{\frac{g_{1}}{2}-1} C_{\mu_{1}^{k}p_{2}^{t}}(x)^{w_{\mu_{1}^{k}p_{2}^{t}}} C_{-\mu_{1}^{k}p_{2}^{t}}(x)^{2^{s}-w_{\mu_{1}^{k}p_{2}^{t}}} \prod\nolimits_{r = 0}^{t-1}\prod\nolimits_{k^{\prime} = 0}^{g_{2, t-r}\gcd(f_{1}, f_{2, t-r})-1} C_{\mu_{2}^{k^{\prime}}p_{1}p_{2}^{r}}(x)^{2^{s-1}} \right). \end{eqnarray*} |
(3) If f_{1} is even and f_{2} is odd, then there exist (2^{s}+1)^{\frac{p_{2}^{t}(p_{1}-1)}{2}} self-dual cyclic codes of length p_{1}p_{2}^{t} over \mathbb{F}_{2^{m}} , which are given by
\begin{eqnarray*} &\left((x-1)^{2^{s-1}}\prod\nolimits_{r = 0}^{t-1}\prod\nolimits_{k_{1} = 0}^{g_{1}-1}\prod\nolimits_{k_{2} = 0}^{\frac{g_{2, t-r}\gcd(f_{1}, f_{2, t-r})}{2}-1} C_{\mu_{1}^{k_{1}}\mu_{2}^{k_{2}}p_{2}^{r}}(x)^{v_{\mu_{1}^{k_{1}}\mu_{2}^{k_{2}}p_{2}^{r}}} C_{-\mu_{1}^{k_{1}}\mu_{2}^{k_{2}}p_{2}^{r}}(x)^{2^{s}-v_{\mu_{1}^{k_{1}}\mu_{2}^{k_{2}}p_{2}^{r}}} \right.\\ &\left.\cdot\prod\nolimits_{k = 0}^{g_{1}-1} C_{\mu_{1}^{k}p_{2}^{t}}(x)^{2^{s-1}} \prod\nolimits_{r = 0}^{t-1}\prod\nolimits_{k^{\prime} = 0}^{\frac{g_{2, t-r}\gcd(f_{1}, f_{2, t-r})}{2}-1} C_{\mu_{2}^{k^{\prime}}p_{1}p_{2}^{r}}(x)^{x_{\mu_{2}^{k^{\prime}}p_{1}p_{2}^{r}}} C_{-\mu_{2}^{k^{\prime}}p_{1}p_{2}^{r}}(x)^{2^{s}-x_{\mu_{2}^{k^{\prime}}p_{1}p_{2}^{r}}}\right). \end{eqnarray*} |
(4) If both f_{1} and f_{2} are even, then we have when v_{2}(f_{1})\neq v_{2}(f_{2}) , there exist (2^{s}+1)^{\frac{(p_{1}-1)(p_{2}^{t}-1)}{2}} self-dual cyclic codes of length p_{1}p_{2}^{t} over \mathbb{F}_{2^{m}} , which are given by
\begin{eqnarray*} &\left((x-1)^{2^{s-1}}\prod\nolimits_{r = 0}^{t-1}\prod\nolimits_{k_{1} = 0}^{g_{1}-1}\prod\nolimits_{k_{2} = 0}^{\frac{g_{2, t-r}\gcd(f_{1}, f_{2, t-r})}{2}-1} C_{\mu_{1}^{k_{1}}\mu_{2}^{k_{2}}p_{2}^{r}}(x)^{v_{\mu_{1}^{k_{1}}\mu_{2}^{k_{2}}p_{2}^{r}}} C_{-\mu_{1}^{k_{1}}\mu_{2}^{k_{2}}p_{2}^{r}}(x)^{2^{s}-v_{\mu_{1}^{k_{1}}\mu_{2}^{k_{2}}p_{2}^{r}}} \right.\\ &\left.\cdot\prod\nolimits_{k = 0}^{g_{1}-1} C_{\mu_{1}^{k}p_{2}^{t}}(x)^{2^{s-1}} \prod\nolimits_{r = 0}^{t-1}\prod\nolimits_{k^{\prime} = 0}^{g_{2, t-r}\gcd(f_{1}, f_{2, t-r})-1} C_{\mu_{2}^{k^{\prime}}p_{1}p_{2}^{r}}(x)^{2^{s-1}} \right). \end{eqnarray*} |
When v_{2}(f_{1}) = v_{2}(f_{2}) , there exist only one self-dual cyclic codes of length p_{1}p_{2}^{t} over \mathbb{F}_{2^{m}} , which is given by
\begin{eqnarray*} &\left((x-1)^{2^{s-1}}\prod\nolimits_{r = 0}^{t-1}\prod\nolimits_{k_{1} = 0}^{g_{1}-1}\prod\nolimits_{k_{2} = 0}^{g_{2, t-r}\gcd(f_{1}, f_{2, t-r})-1} C_{\mu_{1}^{k_{1}}\mu_{2}^{k_{2}}p_{2}^{r}}(x)^{2^{s-1}} \right.\\ &\left.\cdot\prod\nolimits_{k = 0}^{g_{1}-1} C_{\mu_{1}^{k}p_{2}^{t}}(x)^{2^{s-1}} \prod\nolimits_{r = 0}^{t-1}\prod\nolimits_{k^{\prime} = 0}^{g_{2, t-r}\gcd(f_{1}, f_{2, t-r})-1} C_{\mu_{2}^{k^{\prime}}p_{1}p_{2}^{r}}(x)^{2^{s-1}} \right). \end{eqnarray*} |
In this section, we illustrate the above process with the example of constacyclic codes of length 5\ell p^{s} , where \ell is a prime number different from 5 and p . We determine all the constacyclic codes of length 5\ell p^{s} and their dual codes over \mathbb{F}_{q} , and then all the self-dual codes of length 5\ell p^{s} are also given.
First we determine all the q -cyclotomic cosets modulo 5\ell . Let f = \mathrm{ord}_{\ell}(q) , and e = \dfrac{\ell-1}{f} . Then we have:
(1) \mathrm{ord}_{5\ell}(q) = f , when q\equiv 1 \pmod 5 .
(2) \mathrm{ord}_{5\ell}(q) = f , when q\equiv 4 \pmod 5 with f even.
(3) \mathrm{ord}_{5\ell}(q) = 2f , when q\equiv 4 \pmod 5 with f odd.
(4) \mathrm{ord}_{5\ell}(q) = f , when q\equiv 2 or q\equiv 3 \pmod 5 with 4\mid f .
(5) \mathrm{ord}_{5\ell}(q) = 2f , when q\equiv 2 or q\equiv 3 \pmod 5 with 2\mid f but 4\nmid f .
(6) \mathrm{ord}_{5\ell}(q) = 4f , when q\equiv 2 or q\equiv 3 \pmod 5 with f odd.
As the discussion given in the Section 3, we can find a primitive root \mu modulo \ell^{t} for all t\geq 1 such that \mu\equiv 1 \pmod 5 . The following lemma give more explicit formula for the q -cyclotomic cosets modulo 5\ell .
Lemma 6.1.
(1) If q\equiv 1 \pmod 5 , then we have that all the distinct q -cyclotomic cosets modulo 5\ell are given by C_{0} = \{0\} , C_{\ell} = \{\ell\} , C_{2\ell} = \{2\ell\} , C_{-\ell} = \{-\ell\} , C_{-2\ell} = \{-2\ell\} , and C_{a\mu^{k}} = \{a\mu^{k}, a\mu^{k}q, \cdots, a\mu^{k}q^{f-1}\} for a\in R = \{1, 2, -1, -2, 5\} and 0\leq k\leq e-1 .
(2) If q\equiv 4 \pmod 5 and f is even, we have that all the distinct q -cyclotomic cosets modulo 5\ell are given by C_{0} = \{0\} , C_{\ell} = \{\ell, \ell q\} , C_{2\ell} = \{2\ell, 2\ell q\} , C_{\mu^{k'}} = \{\mu^{k'}, \mu^{k'}q, \cdots, \mu^{k'}q^{f-1}\} , C_{2\mu^{k'}} = \{2\mu^{k'}, 2\mu^{k'}q, \cdots, 2\mu^{k'}q^{f-1}\} for 0\leq k'\leq 2e-1 , and C_{5\mu^{k}} = \{5\mu^{k}, 5\mu^{k}q, \cdots, 5\mu^{k}q^{f-1}\} for 0\leq k\leq e-1 .
(3) If q\equiv 4 \pmod 5 and f is odd, we have that all the distinct q -cyclotomic cosets modulo 5\ell are given by C_{0} = \{0\} , C_{\ell} = \{\ell, \ell q\} , C_{2\ell} = \{2\ell, 2\ell q\} , C_{\mu^{k}} = \{\mu^{k}, \mu^{k}q, \cdots, \mu^{k}q^{2f-1}\} , C_{2\mu^{k}} = \{2\mu^{k}, 2\mu^{k}q, \cdots, 2\mu^{k}q^{2f-1}\} , and C_{5\mu^{k}} = \{5\mu^{k}, 5\mu^{k}q, \cdots, 5\mu^{k}q^{f-1}\} for 0\leq k\leq e-1 .
(4) If q\equiv 2 or 3 \pmod 5 and 4\mid f , we have that all the distinct q -cyclotomic cosets modulo 5\ell are given by C_{0} = \{0\} , C_{\ell} = \{\ell, \ell q, \ell q^{2}, \ell q^{3}\} , C_{\mu^{k'}} = \{\mu_{k'}, \mu^{k'}q, \cdots, \mu^{k'}q^{f-1}\} for 0\leq k'\leq 4e-1 , and C_{5\mu^{k}} = \{5\mu^{k}, 5\mu^{k}q, \cdots, 5\mu^{k}q^{f-1}\} for 0\leq k\leq e-1 .
(5) If q\equiv 2 or 3 \pmod 5 and 2\mid f but 4\nmid f , we have that all the distinct q -cyclotomic cosets modulo 5\ell are given by C_{0} = \{0\} , C_{\ell} = \{\ell, \ell q, \ell q^{2}, lq^{3}\} , C_{\mu^{k'}} = \{\mu_{k'}, \mu^{k'}q, \cdots, \mu^{k'}q^{2f-1}\} for 0\leq k'\leq 2e-1 , and C_{5\mu^{k}} = \{5\mu^{k}, 5\mu^{k}q, \cdots, 5\mu^{k}q^{f-1}\} for 0\leq k\leq e-1 .
(6) If q\equiv 2 or 3 \pmod 5 and f is odd, we have that all the distinct q -cyclotomic cosets modulo 5\ell are given by C_{0} = \{0\} , C_{\ell} = \{\ell, \ell q, \ell q^{2}, \ell q^{3}\} , C_{\mu^{k}} = \{\mu_{k}, \mu^{k}q, \cdots, \mu^{k}q^{4f-1}\} , and C_{5\mu^{k}} = \{5\mu^{k}, 5\mu^{k}q, \cdots, 5\mu^{k}q^{f-1}\} for 0\leq k\leq e-1 .
Proof. The methods to prove the above 6 situations are similar, and we will give the proof of the second situation as a instance. First since \mu is a fixed primitive root modulo l such that \mu\equiv 1 \pmod 5 , it is trivial to verify that C_{0} , C_{\ell} , C_{2\ell} , C_{\mu^{k'}} , C_{2\mu^{k'}} for 0\leq k'\leq 2e-1 and C_{5\mu^{k}} for 0\leq k\leq e-1 are q -cyclotomic cosets modulo 5\ell . And then we claim that all these cosets are all distinct. If we have that a_{1}\mu^{k_{1}}\equiv a_{2}\mu^{k_{2}}q^{j} , where a_{1} , a_{2} , k_{1} , k_{2} and j satisfy the definitions in (2). Since
\gcd(a_{1}, 5\ell ) = \gcd(a_{1}\mu^{k_{1}}, 5\ell ) = \gcd(a_{2}\mu^{k_{2}}q^{j}, 5\ell ) = \gcd(a_{2}, 5\ell ), |
we have that either a_{1} = a_{2} or a_{1}\neq a_{2} and both a_{1} and a_{2} are not equal to 5 . We divide the proof into 2 subcases.
Subcase 1. If a_{1} = a_{2} , we have that \mu^{k_{1}-k_{2}}\equiv q^{j}\pmod {\ell} and \mu^{(k_{1}-k_{2})f}\equiv 1\pmod {\ell} , therefore \phi(\ell)\mid (k_{1}-k_{2})f and \dfrac{\phi(\ell)}{f}\mid (k_{1}-k_{2}) , which indicates that k_{1} = k_{2} .
Subcase 2. If a_{1}\neq a_{2} and none of them is equal to 5 , we have that a_{1}a_{2}^{-1}\equiv \mu^{k_{2}-k_{1}}q^{j}\pmod {5\ell} , but notice that a_{1}a_{2}^{-1}\equiv \pm2 \pmod 5 and \mu^{k_{2}-k_{1}}q^{j}\equiv \pm1 \pmod 5 , which is a contradiction. Hence the given cosets are all distinct, and we only need to prove they are all the q -cyclotomic cosets to complete the proof.
Notice that
\vert C_{0}\vert+\vert C_{\ell}\vert+\vert C_{2\ell}\vert+\sum\limits_{k' = 0}^{2e-1}\vert C_{\mu^{k'}}\vert+\sum\limits_{k' = 0}^{2e-1}\vert C_{2\mu^{k'}}\vert+\sum\limits_{k = 0}^{e-1}\vert C_{5\mu^{k}}\vert = 5+2ef+2ef+ef = 5(ef+1) = 5(\phi(\ell)+1) = 5\ell. |
Therefore the conclusion holds.
Theorem 6.1. The irreducible factorization of x^{5\ell }-1 over \mathbb{F}_q is given as follows.
(1) If q\equiv 1 \pmod 5 , then
x^{5\ell }-1 = C_{0}(x)C_{\ell}(x)C_{2\ell}(x)C_{3\ell}(x)C_{4\ell}(x)\prod\limits_{a\in R}\prod\limits_{k = 0}^{e-1}C_{a\mu^{k}}(x), |
where R = {1, 2, 3, 4, 5} .
(2) If q\equiv 4 \pmod 5 and f is even, then
x^{5\ell }-1 = C_{0}(x)C_{\ell}(x)C_{2\ell}(x)\prod\limits_{k' = 0}^{2e-1}C_{\mu^{k'}}(x)C_{2\mu^{k'}}(x)\prod\limits_{k = 0}^{e-1}C_{5\mu^{k}}(x), |
(3) If q\equiv 4 \pmod 5 and f is odd, then
x^{5\ell }-1 = C_{0}(x)C_{\ell}(x)C_{2\ell}(x)\prod\limits_{k = 0}^{e-1}C_{\mu^{k}}(x)C_{2\mu^{k}}(x)C_{5\mu^{k}}(x), |
(4) If q\equiv 2 or 3 \pmod 5 and 4\mid f , then
x^{5\ell }-1 = C_{0}(x)C_{\ell}(x)\prod\limits_{k' = 0}^{4e-1}C_{\mu^{k'}}(x)\prod\limits_{k = 0}^{e-1}C_{5\mu^{k}}(x), |
(5) If q\equiv 2 or 3 \pmod 5 and 2\mid f but 4\nmid f , then
x^{5\ell }-1 = C_{0}(x)C_{\ell}(x)\prod\limits_{k' = 0}^{2e-1}C_{\mu^{k'}}(x)\prod\limits_{k = 0}^{e-1}C_{5\mu^{k}}(x), |
(6) If q\equiv 2 or 3 \pmod 5 and f is odd, then
x^{5\ell }-1 = C_{0}(x)C_{\ell}(x)\prod\limits_{k = 0}^{e-1}C_{\mu^{k}}(x)C_{5\mu^{k}}(x), |
With the irreducible factorization of x^{5\ell }-1 , we can straightly follow the process given in Section 4 to calculate all the constacyclic codes of length 5\ell p^{s} over \mathbb{F}_{q} . We list the result as follow.
Theorem 6.2. Assume that \gcd(q-1, 5\ell p^{s}) = 1 , then \lambda -constacyclic codes C of length 5\ell p^{s} over \mathbb{F}_q are equivalent to the cyclic codes, i.e., for any \lambda\in \mathbb{F}_{q}^* , there exists a unique element a\in \mathbb{F}_{q}^* such that a^{5\ell p^{s}}\lambda = 1 . Furthermore, the irreducible factorization of x^{5\ell p^{s}}-\lambda over \mathbb{F}_q is given by
(1) If q\equiv 4 \pmod 5 and f is even, then
x^{5\ell p^{s}}-\lambda = \widehat{C}_{0}(ax)^{p^{s}}\widehat{C}_{\ell}(ax)^{p^{s}}\widehat{C}_{2\ell}(ax)^{p^{s}}\prod\limits_{k' = 0}^{2e-1}\widehat{C}_{\mu^{k'}} (ax)^{p^{s}}\widehat{C}_{2\mu^{k'}}(ax)^{p^{s}}\prod\limits_{k = 0}^{e-1}\widehat{C}_{5\mu^{k}}(ax)^{p^{s}}, |
Therefore we have that
C = \left(\widehat{C}_{0}(ax)^{\varepsilon_{1}}\widehat{C}_{\ell}(ax)^{\varepsilon_{2}}\widehat{C}_{2\ell}(ax)^{\varepsilon_{3}}\prod\limits_{k' = 0}^{2e-1}\widehat{C}_{\mu^{k'}} (ax)^{\tau_{k^{'}}}\widehat{C}_{2\mu^{k'}}(ax)^{\nu_{k^{'}}}\prod\limits_{k = 0}^{e-1}\widehat{C}_{5\mu^{k}}(ax)^{\rho_{k}}\right), |
and
\begin{eqnarray*} C^{\bot}& = &\left(\widehat{C}_{0}(a^{-1}x)^{p^{s}-\varepsilon_{1}}\widehat{C}_{-\ell}(a^{-1}x)^{p^{s}-\varepsilon_{2}}\widehat{C}_{-2\ell}(a^{-1}x)^{p^{s}-\varepsilon_{3}}\right.\\ &&\times\left.\prod\limits_{k' = 0}^{2e-1}\widehat{C}_{-\mu^{k'}} (a^{-1}x)^{p^{s}-\tau_{k^{'}}}\widehat{C}_{-2\mu^{k'}}(a^{-1}x)^{p^{s}-\nu_{k^{'}}}\prod\limits_{k = 0}^{e-1}\widehat{C}_{-5\mu^{k}}(a^{-1}x)^{p^{s}-\rho_{k}}\right), \end{eqnarray*} |
where 0\leq \varepsilon_{1}, \varepsilon_{2}, \varepsilon_{3}, \tau_{k^{'}}, \nu_{k^{'}}, \rho_{k}\leq p^{s} , for any k^{'} = 0, 1, \cdots, 2e-1 , and k = 0, 1, \cdots, e-1 .
(2) If q\equiv 4 \pmod 5 and f is odd, then
x^{5\ell p^{s}}-\lambda = \widehat{C}_{0}(ax)^{p^{s}}\widehat{C}_{\ell}(ax)^{p^{s}}\widehat{C}_{2\ell}(ax)^{p^{s}}\prod\limits_{k = 0}^{e-1}\widehat{C}_{\mu^{k}}(ax)^{p^{s}}\widehat{C}_{2\mu^{k}} (ax)^{p^{s}}\widehat{C}_{5\mu^{k}}(ax)^{p^{s}}. |
Therefore we have that
C = \left(\widehat{C}_{0}(ax)^{\varepsilon_{1}}\widehat{C}_{\ell}(ax)^{\varepsilon_{2}}\widehat{C}_{2\ell}(ax)^{\varepsilon_{3}}\prod\limits_{k = 0}^{e-1}\widehat{C}_{\mu^{k}}(ax)^{\tau_{k}}\widehat{C}_{2\mu^{k}} (ax)^{\nu_{k}}\widehat{C}_{5\mu^{k}}(ax)^{\rho_{k}}\right), |
and
\begin{eqnarray*} C^{\bot}& = &\left(\widehat{C}_{0}(a^{-1}x)^{p^{s}-\varepsilon_{1}}\widehat{C}_{-\ell}(a^{-1}x)^{p^{s}-\varepsilon_{2}}\widehat{C}_{-2\ell}(a^{-1}x)^{p^{s}-\varepsilon_{3}}\right.\\ &&\times\left.\prod\limits_{k = 0}^{e-1}\widehat{C}_{-\mu^{k}}(a^{-1}x)^{p^{s}-\tau_{k}}\widehat{C}_{-2\mu^{k}} (a^{-1}x)^{p^{s}-\nu_{k}}\widehat{C}_{-5\mu^{k}}(a^{-1}x)^{p^{s}-\rho_{k}}\right), \end{eqnarray*} |
where 0\leq \varepsilon_{1}, \varepsilon_{2}, \varepsilon_{3}, \tau_{k}, \nu_{k}, \rho_{k}\leq p^{s} , for k = 0, 1, \cdots, e-1 .
(3) If q\equiv 2 or 3 \pmod 5 and 4\mid f , then
x^{5\ell p^{s}}-\lambda = \widehat{C}_{0}(ax)^{p^{s}}\widehat{C}_{\ell}(ax)^{p^{s}}\prod\limits_{k' = 0}^{4e-1}\widehat{C}_{\mu^{k'}}(ax)^{p^{s}}\prod\limits_{k = 0}^{e-1}\widehat{C}_{5\mu^{k}}(ax)^{p^{s}}. |
Therefore we have that
C = \left(\widehat{C}_{0}(ax)^{\varepsilon_{1}}\widehat{C}_{\ell}(ax)^{\varepsilon_{2}}\prod\limits_{k' = 0}^{4e-1}\widehat{C}_{\mu^{k'}}(ax)^{\tau_{k^{'}}}\prod\limits_{k = 0}^{e-1}\widehat{C}_{5\mu^{k}}(ax)^{\nu_{k}}\right), |
and
C^{\bot} = \left(\widehat{C}_{0}(a^{-1}x)^{p^{s}-\varepsilon_{1}}\widehat{C}_{-\ell}(a^{-1}x)^{p^{s}-\varepsilon_{2}}\prod\limits_{k' = 0}^{4e-1}\widehat{C}_{-\mu^{k'}}(a^{-1}x)^{p^{s}-\tau_{k^{'}}}\prod\limits_{k = 0}^{e-1}\widehat{C}_{-5\mu^{k}}(a^{-1}x)^{p^{s}-\nu_{k}}\right), |
where 0\leq \varepsilon_{1}, \varepsilon_{2}, \tau_{k^{'}}, \nu_{k}\leq p^{s} , for k^{'} = 0, 1, \cdots, 4e-1 , and k = 0, 1, \cdots, e-1 .
(4) If q\equiv 2 or 3 \pmod 5 and 2\mid f but 4\nmid f , then
x^{5\ell p^{s}}-\lambda = \widehat{C}_{0}(ax)^{p^{s}}\widehat{C}_{\ell}(ax)^{p^{s}}\prod\limits_{k' = 0}^{2e-1}\widehat{C}_{\mu^{k'}}(ax)^{p^{s}}\prod\limits_{k = 0}^{e-1}\widehat{C}_{5\mu^{k}}(ax)^{p^{s}}. |
Therefore we have that
C = \left(\widehat{C}_{0}(ax)^{\varepsilon_{1}}\widehat{C}_{\ell}(ax)^{\varepsilon_{2}}\prod\limits_{k' = 0}^{2e-1}\widehat{C}_{\mu^{k'}}(ax)^{\tau_{k^{'}}}\prod\limits_{k = 0}^{e-1}\widehat{C}_{5\mu^{k}}(ax)^{\nu_{k}}\right), |
and
C^{\bot} = \left(\widehat{C}_{0}(a^{-1}x)^{p^{s}-\varepsilon_{1}}\widehat{C}_{-\ell}(a^{-1}x)^{p^{s}-\varepsilon_{2}}\prod\limits_{k' = 0}^{2e-1}\widehat{C}_{-\mu^{k'}}(a^{-1}x)^{p^{s}-\tau_{k^{'}}}\prod\limits_{k = 0}^{e-1}\widehat{C}_{-5\mu^{k}}(a^{-1}x)^{p^{s}-\nu_{k}}\right), |
where 0\leq \varepsilon_{1}, \varepsilon_{2}, \tau_{k^{'}}, \nu_{k}\leq p^{s} , for k^{'} = 0, 1, \cdots, 2e-1 , and k = 0, 1, \cdots, e-1 .
(5) If q\equiv 2 or 3 \pmod 5 and f is odd, then
x^{5\ell p^{s}}-\lambda = \widehat{C}_{0}(ax)^{p^{s}}\widehat{C}_{\ell}(ax)^{p^{s}}\prod\limits_{k = 0}^{e-1}\widehat{C}_{\mu^{k}}(ax)^{p^{s}}\widehat{C}_{5\mu^{k}}(ax)^{p^{s}}. |
Therefore we have that
C = \left(\widehat{C}_{0}(ax)^{\varepsilon_{1}}\widehat{C}_{\ell}(ax)^{\varepsilon_{2}}\prod\limits_{k = 0}^{e-1}\widehat{C}_{\mu^{k}}(ax)^{\tau_{k}}\widehat{C}_{5\mu^{k}}(ax)^{\nu_{k}}\right), |
and
C^{\bot} = \left(\widehat{C}_{0}(a^{-1}x)^{p^{s}-\varepsilon_{1}}\widehat{C}_{-\ell}(a^{-1}x)^{p^{s}-\varepsilon_{2}}\prod\limits_{k = 0}^{e-1}\widehat{C}_{-\mu^{k}}(a^{-1}x)^{p^{s}-\tau_{k}}\widehat{C}_{-5\mu^{k}}(a^{-1}x)^{p^{s}-\nu_{k}}\right), |
where 0\leq \varepsilon_{1}, \varepsilon_{2}, \tau_{k}, \nu_{k}\leq p^{s} , for k = 0, 1, \cdots, e-1 .
Theorem 6.3. Assume that \gcd(q-1, 5\ell p^{s}) = 5\ell , then \mathbb{F}_{q}^* = \langle\xi\rangle = \langle\xi^{5\ell }\rangle\cup \langle\xi^{5\ell }\rangle\xi^{p^{s}}\cup \cdots\cup \langle\xi^{5\ell }\rangle\xi^{p^{s}(5\ell -1)} . For any \lambda\in \mathbb{F}_{q}^* , there exists an element a\in \mathbb{F}_{q}^* such that a^{5\ell p^{s}}\lambda = \xi^{j\cdot p^{s}} , where 0\leq j\leq 5\ell -1 . Then j can be written as j = y\cdot 5^{v_{1}}\ell^{v_{2}} , where v_{1} = \min\{1, v_{5}(j)\} and v_{2} = \min\{1, v_{\ell}(j)\} . And
\begin{eqnarray*} x^{n}-\lambda& = &(x^{5^{1-v_{1}}\ell^{1-v_{2}}}-a^{-5^{1-v_{1}}\ell^{1-v_{2}}}\xi^{y})^{p^{s}}(x^{5^{1-v_{1}}\ell^{1-v_{2}}}-a^{-5^{1-v_{1}}\ell^{1-v_{2}}}\delta\xi^{y})^{p^{s}}\\ &&\cdots(x^{5^{1-v_{1}}\ell^{1-v_{2}}}-a^{-5^{1-v_{1}}\ell^{1-v_{2}}}\delta^{5^{v_{1}}\ell^{v_{2}}-1}\xi^{y})^{p^{s}} \end{eqnarray*} |
gives the irreducible factorization of x^{5\ell p^{s}}-\lambda over \mathbb{F}_{q} . Moreover, all the \lambda -constacyclic codes of length 5lp^{s} and their dual codes are given by
\begin{eqnarray*} C& = &\left((x^{5^{1-v_{1}}\ell^{1-v_{2}}}-a^{-5^{1-v_{1}}\ell^{1-v_{2}}}\xi^{y})^{\varepsilon_{1}}(x^{5^{1-v_{1}}\ell^{1-v_{2}}}-a^{-5^{1-v_{1}}\ell^{1-v_{2}}}\delta\xi^{y})^{\varepsilon_{2}}\right.\\ &&\left.\cdots(x^{5^{1-v_{1}}\ell^{1-v_{2}}}-a^{-5^{1-v_{1}}\ell^{1-v_{2}}}\delta^{5^{v_{1}}\ell^{v_{2}}-1}\xi^{y})^{\varepsilon_{5^{v_{1}}\ell^{v_{2}}}}\right), \end{eqnarray*} |
and
\begin{eqnarray*} C^{\bot}& = &\left((x^{5^{1-v_{1}}\ell^{1-v_{2}}}-a^{5^{1-v_{1}}\ell^{1-v_{2}}}\xi^{-y})^{p^{s}-\varepsilon_{1}}(x^{5^{1-v_{1}}\ell^{1-v_{2}}}-a^{5^{1-v_{1}}\ell^{1-v_{2}}}\delta^{-1}\xi^{-y})^{p^{s}-\varepsilon_{2}}\right.\\ &&\left.\cdots(x^{5^{1-v_{1}}\ell^{1-v_{2}}}-a^{5^{1-v_{1}}\ell^{1-v_{2}}}\delta^{1-5^{v_{1}}\ell^{v_{2}}}\xi^{-y})^{p^{s}-\varepsilon_{5^{v_{1}}\ell^{v_{2}}}}\right), \end{eqnarray*} |
where 0\leq \varepsilon_{1}, \varepsilon_{2}, \cdots, \varepsilon_{5^{v_{1}}\ell^{v_{2}}}\leq p^{s} .
Theorem 6.4. Assume that \gcd(q-1, 5\ell p^{s}) = 5 , then for any 0\leq j\leq 4 , there exists an element a\in \mathbb{F}_{q^{f}}* such that a^{5\ell p^{s}}\xi^{j\cdot p^{s}} = \zeta^{j'\cdot p^{s}} . Moreover, each irreducible factor of x^{5\ell}-\xi^{j} over \mathbb{F}_{q} is of the form
\begin{eqnarray*} &&(x^{5^{1-v_{1}}\ell^{1-v_{2}}}-a^{-5^{1-v_{1}}\ell^{1-v_{2}}}\delta^{i}\zeta^{y'})(x^{5^{1-v_{1}}\ell^{1-v_{2}}}-a^{-5^{1-v_{1}}\ell^{1-v_{2}}\cdot q}\delta^{iq}\zeta^{y'q})\\ &&\cdots(x^{5^{1-v_{1}}\ell^{1-v_{2}}}-a^{-5^{1-v_{1}}\ell^{1-v_{2}}\cdot q^{z_{i}-1}}\delta^{iq^{z_{i}-1}}\zeta^{y'q^{z_{i}-1}}), \end{eqnarray*} |
where j' = y'5^{v_{1}}\ell^{v_{2}} , v_{1} = \min\{1, v_{5}(j')\} , v_{2} = \min\{1, v_{\ell}(j')\} , and z_{i} is the least positive integer such that a^{-q^{z_{i}}5^{1-v_{1}}\ell^{1-v_{2}}}\delta^{iq^{z_{i}}}\zeta^{y'q^{z_{i}}} = a^{5^{1-v_{1}}\ell^{1-v_{2}}}\delta^{i}\zeta^{y'} .
For any 0\leq i, i^{'}\leq 5^{v_{1}}\ell^{v_{2}}-1 , we define a relation \sim to be such that i\sim i^{'} if and only if a^{-q^{m}5^{1-v_{1}}\ell^{1-v_{2}}}\delta^{iq^{m}}\zeta^{y'q^{m}} = a^{5^{1-v_{1}}\ell^{1-v_{2}}}\delta^{i^{'}}\zeta^{y'} for some nonnegative integers m . It is obvious to see that \sim is an equivalence relation. Assume that S is a complete system of equivalence class representatives of \{0, 1, \cdots, 5^{v_{1}}\ell^{v_{2}}-1\} relative to this relation \sim . For any i\in S we denote the irreducible polynomial
\begin{eqnarray*} &&(x^{5^{1-v_{1}}\ell^{1-v_{2}}}-a^{-5^{1-v_{1}}\ell^{1-v_{2}}}\delta^{i}\zeta^{y'})(x^{5^{1-v_{1}}\ell^{1-v_{2}}}-a^{-5^{1-v_{1}}\ell^{1-v_{2}}\cdot q}\delta^{iq}\zeta^{y'q})\\ &&\cdots(x^{5^{1-v_{1}}\ell^{1-v_{2}}}-a^{-5^{1-v_{1}}\ell^{1-v_{2}}\cdot q^{z_{i}-1}}\delta^{iq^{z_{i}-1}}\zeta^{y'q^{z_{i}-1}}), \end{eqnarray*} |
by M_{i}(x) , and denote
\begin{eqnarray*} &&(x^{5^{1-v_{1}}\ell^{1-v_{2}}}-a^{5^{1-v_{1}}\ell^{1-v_{2}}}\delta^{-i}\zeta^{-y'})(x^{5^{1-v_{1}}\ell^{1-v_{2}}}-a^{5^{1-v_{1}}\ell^{1-v_{2}}\cdot q}\delta^{-iq}\zeta^{-y'q})\\ &&\cdots(x^{5^{1-v_{1}}\ell^{1-v_{2}}}-a^{5^{1-v_{1}}\ell^{1-v_{2}}\cdot q^{z_{i}-1}}\delta^{-iq^{z_{i}-1}}\zeta^{-y'q^{z_{i}-1}}), \end{eqnarray*} |
by M_{i}^{'}(x) . Then we have the following corollary.
Corollary 6.1. Assume that \gcd(q-1, 5\ell p^{s}) = 5 . For any 0\leq j\leq 4 , there exists an element a\in \mathbb{F}_{q^{f}}* such that a^{5\ell p^{s}}\xi^{j\cdot p^{s}} = \zeta^{j'\cdot p^{s}} . Then
x^{5\ell p^{s}}-\xi^{j p^{s}} = \prod\limits_{i\in S}M_{i}(x)^{p^{s}} |
gives the irreducible factorization of x^{5\ell p^{s}}-\xi^{j p^{s}} over \mathbb{F}_q . Furthermore we have that
C = \left(\prod\limits_{i\in X}M_{i}(x)^{\varepsilon_{i}}\right), |
and
C^{\bot} = \left(\prod\limits_{i\in X}M_{i}^{'}(x)^{p^{s}-\varepsilon_{i}}\right), |
where 0\leq \varepsilon_{i}\leq p^{s} , for i\in X .
Theorem 6.5. Assume that \gcd(q-1, 5\ell p^{s}) = \ell , then
(1) If q\equiv 4 \pmod 5 , for any 0\leq j\leq \ell-1 , the following equations
j'\equiv 2j\pmod{\ell}\;\mathit{\text{ and }}\;j'\equiv 0 \pmod 5 |
have a unique solution j' up to modulo 5\ell . Moreover, each irreducible facotor of x^{5\ell}-\xi^{j} over \mathbb{F}_{q} is of the form
\begin{eqnarray*} &&(x^{5^{1-v_{1}}\ell^{1-v_{2}}}-a^{-5^{1-v_{1}}\ell^{1-v_{2}}}\delta^{i}\zeta^{y'})(x^{5^{1-v_{1}}\ell^{1-v_{2}}}-a^{-5^{1-v_{1}}\ell^{1-v_{2}}\cdot q}\delta^{iq}\zeta^{y'q})\\ &&\cdots(x^{5^{1-v_{1}}\ell^{1-v_{2}}}-a^{-5^{1-v_{1}}\ell^{1-v_{2}}\cdot q^{z_{i}-1}}\delta^{iq^{z_{i}-1}}\zeta^{y'q^{z_{i}-1}}), \end{eqnarray*} |
where j' = y'5^{v_{1}}\ell^{v_{2}} , v_{1} = \min\{1, v_{5}(j')\} , v_{2} = \min\{1, v_{\ell}(j')\} , and z_{i} is the least positive integer such that a^{-q^{z_{i}}5^{1-v_{1}}\ell^{1-v_{2}}}\delta^{iq^{z_{i}}}\zeta^{y'q^{z_{i}}} = a^{5^{1-v_{1}}\ell^{1-v_{2}}}\delta^{i}\zeta^{y'} .
(2) If q\equiv 2, 3 \pmod 5 , for any 0\leq j\leq \ell-1 , the following equations
j'\equiv 4j \pmod{\ell} |
j'\equiv 0 \pmod 5 |
have a unique solution j' up to modulo 5\ell . Moreover, each irreducible facotor of x^{5\ell}-\xi^{j} over \mathbb{F}_{q} is of the form
\begin{eqnarray*} &&(x^{5^{1-v_{1}}\ell^{1-v_{2}}}-a^{-5^{1-v_{1}}\ell^{1-v_{2}}}\delta^{i}\zeta^{y'})(x^{5^{1-v_{1}}\ell^{1-v_{2}}}-a^{-5^{1-v_{1}}\ell^{1-v_{2}}\cdot q}\delta^{iq}\zeta^{y'q})\\ &&\cdots(x^{5^{1-v_{1}}\ell^{1-v_{2}}}-a^{-5^{1-v_{1}}\ell^{1-v_{2}}\cdot q^{z_{i}-1}}\delta^{iq^{z_{i}-1}}\zeta^{y'q^{z_{i}-1}}), \end{eqnarray*} |
where j' = y'5^{v_{1}}\ell^{v_{2}} , v_{1} = min{1, v_{5}(j')} , v_{2} = min{1, v_{\ell}(j')} , and z_{i} is the least positive integer such that a^{-q^{z_{i}}5^{1-v_{1}}\ell^{1-v_{2}}}\delta^{iq^{z_{i}}}\zeta^{y'q^{z_{i}}} = a^{5^{1-v_{1}}\ell^{1-v_{2}}}\delta^{i}\zeta^{y'} .
For any 0\leq i, i^{'}\leq 5^{v_{1}}\ell^{v_{2}}-1 , we define a relation \sim to be such that i\sim i^{'} if and only if a^{-q^{m}5^{1-v_{1}}\ell^{1-v_{2}}}\delta^{iq^{m}}\zeta^{y'q^{m}} = a^{5^{1-v_{1}}\ell^{1-v_{2}}}\delta^{i^{'}}\zeta^{y'} for some nonnegative integer m . It is obvious to see that \sim is an equivalence relation. Assume that S is a complete system of equivalence class representatives of \{0, 1, \cdots, 5^{v_{1}}\ell^{v_{2}}-1\} relative to this relation \sim . For any i\in S we denote the irreducible polynomial
\begin{eqnarray*} &&(x^{5^{1-v_{1}}\ell^{1-v_{2}}}-a^{-5^{1-v_{1}}\ell^{1-v_{2}}}\delta^{i}\zeta^{y'})(x^{5^{1-v_{1}}\ell^{1-v_{2}}}-a^{-5^{1-v_{1}}\ell^{1-v_{2}}\cdot q}\delta^{iq}\zeta^{y'q})\\ &&\cdots(x^{5^{1-v_{1}}\ell^{1-v_{2}}}-a^{-5^{1-v_{1}}\ell^{1-v_{2}}\cdot q^{z_{i}-1}}\delta^{iq^{z_{i}-1}}\zeta^{y'q^{z_{i}-1}}), \end{eqnarray*} |
by M_{i}(x) , and denote
\begin{eqnarray*} &&(x^{5^{1-v_{1}}\ell^{1-v_{2}}}-a^{5^{1-v_{1}}\ell^{1-v_{2}}}\delta^{-i}\zeta^{-y'})(x^{5^{1-v_{1}}\ell^{1-v_{2}}}-a^{5^{1-v_{1}}\ell^{1-v_{2}}\cdot q}\delta^{-iq}\zeta^{-y'q})\\ &&\cdots(x^{5^{1-v_{1}}\ell^{1-v_{2}}}-a^{5^{1-v_{1}}\ell^{1-v_{2}}\cdot q^{z_{i}-1}}\delta^{-iq^{z_{i}-1}}\zeta^{-y'q^{z_{i}-1}}), \end{eqnarray*} |
by M_{i}^{'}(x) .
Corollary 6.2. Assume that \gcd(q-1, 5\ell p^{s}) = \ell , then
(1) If q\equiv 4 \pmod 5 , and j, j^{'} is defined as in the first case of Theorem 5.1, then
x^{5\ell p^{s}}-\xi^{j p^{s}} = \prod\limits_{i\in X}M_{i}(x)^{p^{s}} |
gives the irreducible factorization of x^{5\ell p^{s}}-\xi^{j p^{s}} over \mathbb{F}_q . Furthermore we have that
C = \left(\prod\limits_{i\in X}M_{i}(x)^{\varepsilon_{i}}\right), |
and
C^{\bot} = \left(\prod\limits_{i\in X}M_{i}^{'}(x)^{p^{s}-\varepsilon_{i}}\right), |
where 0\leq \varepsilon_{i}\leq p^{s} , for i\in X .
(2) If q\equiv 2, 3 \pmod 5 , and j, j^{'} is defined as in the second case of Theorem 5.1, then
x^{5\ell p^{s}}-\xi^{j p^{s}} = \prod\limits_{i\in X}M_{i}(x)^{p^{s}} |
gives the irreducible factorization of x^{5\ell p^{s}}-\xi^{j p^{s}} over \mathbb{F}_q . Furthermore we have that
C = \left(\prod\limits_{i\in X}M_{i}(x)^{\varepsilon_{i}}\right), |
and
C^{\bot} = \left(\prod\limits_{i\in X}M_{i}^{'}(x)^{p^{s}-\varepsilon_{i}}\right), |
where 0\leq \varepsilon_{i}\leq p^{s} , for i\in X .
Finally we give all the self-dual constacylic codes of length 5\ell p^{s} as the end of this section. Since self-dual cyclic codes of length N over \mathbb{F}_q exists if and only if N is even and the characteristic of \mathbb{F}_q is p = 2 , as in the general case, we only consider the case of self-dual cyclic codes of length 5\cdot 2^{s}\ell over \mathbb{F}_{2^{m}} .
Lemma 6.2. Assume that q\equiv 1 \pmod 5 . For the q -cyclotomic cosets, one of the following holds.
(1) If f = \mathrm{ord}_{\ell}(q) is even, we have that
C_{0}^{*} = C_{0}, \ C_{\ell}^{*} = C_{-\ell}, \ C_{2\ell}^{*} = C_{-2\ell}, \ C_{\mu^{k}}^{*} = C_{-\mu^{k}}, \ C_{2\mu^{k}}^{*} = C_{-2\mu^{k}}, \ C_{5\mu^{k}}^{*} = C_{5\mu^{k}}, |
where 0\leq k\leq e-1 .
(2) If f = \mathrm{ord}_{\ell}(q) is odd, we have that
C_{0}^{*} = C_{0}, \ C_{\ell}^{*} = C_{-\ell}, \ C_{2\ell}^{*} = C_{-2\ell}, \ C_{\mu^{k}}^{*} = C_{-\mu^{k}}, \ C_{2\mu^{k}}^{*} = C_{-2\mu^{k}}, \ C_{5\mu^{k^{'}}}^{*} = C_{-5\mu^{k^{'}}}, |
where \{C_{5\mu^{k}}\} = \{C_{5\mu^{k^{'}}}\}\bigcup \{C_{-5\mu^{k^{'}}}\} , and 0\leq k\leq e-1 , 0\leq k^{'}\leq \dfrac{e}{2}-1 .
Proof.
(1) By the definition of reciprocal coset, it is clear that C_{0}^{*} = C_{0}, \ C_{\ell}^{*} = C_{-\ell}, \ C_{2\ell}^{*} = C_{-2\ell}, \ C_{\mu^{k}}^{*} = C_{-\mu^{k}}, \ C_{2\mu^{k}}^{*} = C_{-2\mu^{k}} , thus it remains to prove C_{5\mu^{k}}^{*} = C_{5\mu^{k}} . Let t = \frac{f}{2} . Since f = \mathrm{ord}_{\ell}(q) , it is trivial to see that q^{t}\equiv -1 \pmod \ell , and therefore we have that -5\mu^{k}\equiv 5\mu^{k}q^{t} \pmod {5\ell} . It follows immediately that C_{5\mu^{k}}^{*} = C_{5\mu^{k}} , for 0\leq k\leq e-1 .
(2) As in the first case, the conclusions that C_{0}^{*} = C_{0}, \ C_{\ell}^{*} = C_{-\ell}, \ C_{2\ell}^{*} = C_{-2\ell}, \ C_{\mu^{k}}^{*} = C_{-\mu^{k}}, \ C_{2\mu^{k}}^{*} = C_{-2\mu^{k}} are clear, and now we prove that C_{5\mu^{k^{'}}}^{*} = C_{-5\mu^{k^{'}}} . To see this, we claim that for any 0\leq k_{1}^{'}, k_{2}^{'}\leq \frac{e}{2}-1 , C_{5\mu^{k_{1}^{'}}}\neq C_{-5\mu^{k_{2}^{'}}} , and \{C_{5\mu^{k}}\} = \{C_{5\mu^{k^{'}}}\}\bigcup \{C_{-5\mu^{k^{'}}}\} . Assume that C_{5\mu^{k_{1}^{'}}} = C_{-5\mu^{k_{2}^{'}}} for some 0\leq k_{1}^{'}, k_{2}^{'}\leq \frac{e}{2}-1 , then we have that 5\mu^{k_{1}^{'}}\equiv -5\mu^{k_{2}^{'}}q^{j} \pmod {5\ell} for some 0\leq j\leq f-1 , which indicates that -\mu^{k_{1}^{'}-k_{2}^{'}}\equiv q^{j} \pmod \ell . Notice that f is odd, therefore we have that -\mu^{f(k_{1}^{'}-k_{2}^{'})}\equiv q^{jf} \equiv 1 \pmod \ell and \mu^{f(k_{1}^{'}-k_{2}^{'})}\equiv -1 \pmod \ell . It follows that \mu^{2f(k_{1}^{'}-k_{2}^{'})}\equiv 1 \pmod \ell , hence \phi(\ell)\mid 2f(k_{1}^{'}-k_{2}^{'}) and \frac{e}{2}\mid k_{1}^{'}-k_{2}^{'} . Since by the condition we have 0\leq k_{1}^{'}, k_{2}^{'}\leq \frac{e}{2}-1 , we deduce that k_{1}^{'} = k_{2}^{'} . Then the equation 5\mu^{k_{1}^{'}}\equiv -5\mu^{k_{2}^{'}}q^{j} \pmod {5\ell} can be reduced to -1\equiv q^{j} \pmod \ell . However, notice that \mathrm{ord}_{\ell}(q) = f is odd, such a positive integer j cannot exist, which is a contradiction. According to this, we have that for any 0\leq k_{1}^{'}, k_{2}^{'}\leq \frac{e}{2}-1 , C_{5\mu^{k_{1}^{'}}}\neq C_{-5\mu^{k_{2}^{'}}} . By comparing the number of elements, it is trivial to verify that \{C_{5\mu^{k}}\} = \{C_{5\mu^{k^{'}}}\}\bigcup \{C_{-5\mu^{k^{'}}}\} holds. Then by the definition of reciprocal coset, one immediately get that C_{5\mu^{k^{'}}}^{*} = C_{-5\mu^{k^{'}}} .
With the same method we can prove the results for the rest of cases. The proofs will be omitted.
Lemma 6.3. Assume that q\equiv 4 \pmod 5 . For the q -cyclotomic cosets, one of the following holds.
(1) If f = 2t is even, then
(i) when t is even, we have that
C_{0}^{*} = C_{0}, \ C_{\ell}^{*} = C_{\ell}, \ C_{2\ell}^{*} = C_{2\ell}, \ C_{\mu^{k}}^{*} = C_{-\mu^{k}}, \ C_{2\mu^{k}}^{*} = C_{-2\mu^{k}}, \ C_{5\mu^{k}}^{*} = C_{5\mu^{k}}, |
where \{C_{\mu^{k^{'}}}\} = \{C_{\mu^{k}}\}\bigcup \{C_{-\mu^{k}}\} , \{C_{2\mu^{k^{'}}}\} = \{C_{2\mu^{k}}\}\bigcup \{C_{-2\mu^{k}}\} , for 0\leq k\leq e-1 , 0\leq k^{'}\leq 2e-1 .
(ii) If t is odd, we have that
C_{0}^{*} = C_{0}, \ C_{\ell}^{*} = C_{\ell}, \ C_{2\ell}^{*} = C_{2\ell}, \ C_{\mu^{k^{'}}}^{*} = C_{\mu^{k^{'}}}, \ C_{2\mu^{k^{'}}}^{*} = C_{2\mu^{k^{'}}}, \ C_{5\mu^{k}}^{*} = C_{5\mu^{k}}, |
where 0\leq k\leq e-1 , 0\leq k^{'}\leq 2e-1 .
(2) when f is odd, then
C_{0}^{*} = C_{0}, \ C_{\ell}^{*} = C_{\ell}, \ C_{2\ell}^{*} = C_{2\ell}, \ C_{\mu^{k^{'}}}^{*} = C_{-\mu^{k^{'}}}, \ C_{2\mu^{k^{'}}}^{*} = C_{-2\mu^{k^{'}}}, \ C_{5\mu^{k^{'}}}^{*} = C_{-5\mu^{k^{'}}}, |
where \{C_{\mu^{k}}\} = \{C_{\mu^{k^{'}}}\}\bigcup \{C_{-\mu^{k^{'}}}\} , \{C_{2\mu^{k}}\} = \{C_{2\mu^{k^{'}}}\}\bigcup \{C_{-2\mu^{k^{'}}}\} , \{C_{5\mu^{k}}\} = \{C_{5\mu^{k^{'}}}\}\bigcup \{C_{-5\mu^{k^{'}}}\} , for 0\leq k\leq e-1, 0\leq k^{'}\leq \frac{e}{2}-1 .
Lemma 6.4. Assume that q\equiv 2 or 3 \pmod 5 . For the q -cyclotomic cosets, one of the following holds.
(1) If 4\mid f . Let f = 4t , then
(i) when t is even, we have that
C_{0}^{*} = C_{0}, \ C_{\ell}^{*} = C_{\ell}, \ C_{\mu^{k^{''}}}^{*} = C_{-\mu^{k^{''}}}, \ C_{5\mu^{k}}^{*} = C_{5\mu^{k}}, |
where \{C_{\mu^{k^{'}}}\} = \{C_{\mu^{k^{''}}}\}\bigcup \{C_{-\mu^{k^{''}}}\} , for 0\leq k\leq e-1 , 0\leq k^{''}\leq 2e-1 and 0\leq k^{'}\leq 4e-1 .
(ii) If t is odd, we have that
C_{0}^{*} = C_{0}, \ C_{\ell}^{*} = C_{\ell}, \ C_{\mu^{k^{'}}}^{*} = C_{\mu^{k^{'}}}, \ C_{5\mu^{k}}^{*} = C_{5\mu^{k}}, |
where 0\leq k\leq e-1 , 0\leq k^{'}\leq 4e-1 .
(2) If 2\mid f but 4\nmid f , then
C_{0}^{*} = C_{0}, \ C_{\ell}^{*} = C_{\ell}, \ C_{\mu^{k}}^{*} = C_{-\mu^{k}}, \ C_{5\mu^{k}}^{*} = B_{5\mu^{k}}, |
where \{C_{\mu^{k^{'}}}\} = \{C_{\mu^{k}}\}\bigcup \{C_{-\mu^{k}}\} , for 0\leq k\leq e-1 , 0\leq k^{'}\leq 2e-1 .
(3) If f is odd, then
C_{0}^{*} = C_{0}, \ C_{\ell}^{*} = C_{\ell}, \ C_{\mu^{k^{'}}}^{*} = C_{-\mu^{k^{'}}}, \ C_{5\mu^{k^{'}}}^{*} = C_{-5\mu^{k^{'}}}, |
where \{C_{\mu^{k}}\} = \{C_{\mu^{k^{'}}}\}\bigcup \{C_{-\mu^{k^{'}}}\} , \{C_{5\mu^{k}}\} = \{C_{5\mu^{k^{'}}}\}\bigcup \{C_{-5\mu^{k^{'}}}\} , for 0\leq k^{'}\leq \frac{e}{2}-1 , 0\leq k\leq e-1 .
From the above lemmas, we give all the self-dual cyclic codes of length 5\cdot 2^{s}\ell over \mathbb{F}_{2^{m}} and their enumeration in the following theorems.
Theorem 6.6. Let q\equiv 1 \pmod 5 , then one of the following holds.
(1) If f = \mathrm{ord}_{\ell}(q) is even, there exist (2^{s}+1)^{2+2e} self-dual cyclic codes of length 5\cdot 2^{s}\ell over \mathbb{F}_{2^{m}} , which are given by
\begin{eqnarray*} &&\left((x-1)^{2^{s-1}}C_{\ell}(x)^{\varepsilon_{1}}C_{-\ell}(x)^{2^{s}-\varepsilon_{1}}C_{2\ell}(x)^{\varepsilon_{2}}C_{-2\ell}(x)^{2^{s}-\varepsilon_{2}}\right.\\ &&\times\left.\prod\limits_{k = 0}^{e-1}C_{\mu^{k}}(x)^{\tau_{k}}C_{-\mu^{k}}(x)^{2^{s}-\tau_{k}}C_{2\mu^{k}}(x)^{\rho_{k}}C_{-2\mu^{k}}(x)^{2^{s}-\rho_{k}}C_{5\mu^{k}}(x)^{2^{s-1}}\right), \end{eqnarray*} |
where 0\leq \varepsilon_{1}, \varepsilon_{2}, \tau_{k}, \rho_{k}\leq 2^{s} , for any 0\leq k\leq e-1 .
(2) If f = \mathrm{ord}_{\ell}(q) is odd, there exist (2^{s}+1)^{2+\dfrac{5e}{2}} self-dual cyclic codes of length 5\cdot 2^{s}\ell over \mathbb{F}_{2^{m}} , which are given by
\begin{eqnarray*} &&\left((x-1)^{2^{s-1}}C_{\ell}(x)^{\varepsilon_{1}}C_{-\ell}(x)^{2^{s}-\varepsilon_{1}}C_{2\ell}(x)^{\varepsilon_{2}}C_{-2\ell}(x)^{2^{s}-\varepsilon_{2}}\right.\\ &&\cdot\left.\prod\limits_{k = 0}^{e-1}C_{\mu^{k}}(x)^{\tau_{k}}C_{-\mu^{k}}(x)^{2^{s}-\tau_{k}}C_{2\mu^{k}}(x)^{\rho_{k}}C_{-2\mu^{k}}(x)^{2^{s}-\rho_{k}}\prod\limits_{k^{'} = 0}^{\frac{e}{2}-1}C_{5\mu^{k^{'}}}(x)^{\iota_{k^{'}}}C_{-5\mu^{k^{'}}}(x)^{2^{s}-\iota_{k^{'}}}\right), \end{eqnarray*} |
where 0\leq \varepsilon_{1}, \varepsilon_{2}, \tau_{k}, \rho_{k}, \iota_{k^{'}}\leq 2^{s} , for any 0\leq k\leq e-1 and any 0\leq k^{'}\leq \dfrac{e}{2}-1 .
Proof.
(1) By Lemma 6.2, any self-dual cyclic codes of length 5\cdot 2^{s}\ell over \mathbb{F}_{2^{m}} has the form of
\begin{eqnarray*} &&\left((x-1)^{2^{s-1}}C_{\ell}(x)^{\varepsilon_{1}}C_{-\ell}(x)^{2^{s}-\varepsilon_{1}}C_{2\ell}(x)^{\varepsilon_{2}}C_{-2\ell}(x)^{2^{s}-\varepsilon_{2}}\right.\\ &&\times\left.\prod\limits_{k = 0}^{e-1}C_{\mu^{k}}(x)^{\tau_{k}}C_{-\mu^{k}}(x)^{2^{s}-\tau_{k}}C_{2\mu^{k}}(x)^{\rho_{k}}C_{-2\mu^{k}}(x)^{2^{s}-\rho_{k}}C_{5\mu^{k}}(x)^{2^{s-1}}\right), \end{eqnarray*} |
where 0\leq \varepsilon_{1}, \varepsilon_{2}, \tau_{k}, \rho_{k}\leq 2^{s} , for any 0\leq k\leq e-1 . Since each of \varepsilon_{1}, \varepsilon_{2} and \tau_{k}, \rho_{k} , 0\leq k\leq e-1 , has 2^{s}+1 possible values, we have in total (2^{s}+1)^{2+2e} self-dual cyclic codes of length 5\cdot 2^{s}\ell over \mathbb{F}_{2^{m}} .
(2) By Lemma 6.2, any self-dual cyclic codes of length 5\cdot 2^{s}\ell over \mathbb{F}_{2^{m}} has the form of
\begin{eqnarray*} &&\left((x-1)^{2^{s-1}}C_{\ell}(x)^{\varepsilon_{1}}C_{-\ell}(x)^{2^{s}-\varepsilon_{1}}C_{2\ell}(x)^{\varepsilon_{2}}C_{-2\ell}(x)^{2^{s}-\varepsilon_{2}}\right.\\ &&\cdot\left.\prod\limits_{k = 0}^{e-1}C_{\mu g^{k}}(x)^{\tau_{k}}C_{-\mu^{k}}(x)^{2^{s}- \tau_{k}}C_{2\mu^{k}}(x)^{\rho_{k}}C_{-2\mu^{k}}(x)^{2^{s}-\rho_{k}}\prod\limits_{k^{'} = 0}^{\frac{e}{2}-1}C_{5\mu^{k^{'}}}(x)^{\iota_{k^{'}}}C_{-5\mu^{k^{'}}}(x)^{2^{s}-\iota_{k^{'}}}\right), \end{eqnarray*} |
where 0\leq \varepsilon_{1}, \varepsilon_{2}, \tau_{k}, \rho_{k}, \iota_{k^{'}}\leq 2^{s} , for any 0\leq k\leq e-1 and any 0\leq k^{'}\leq \dfrac{e}{2}-1 . Each of \varepsilon_{1}, \varepsilon_{2} , \tau_{k}, \rho_{k}, 0\leq k\leq e-1 , and \iota_{k^{'}} , 0\leq k^{'}\leq \dfrac{e}{2}-1 , has 2^{s}+1 possible values, we have in total (2^{s}+1)^{2+\dfrac{5e}{2}} self-dual cyclic codes of length 5\cdot 2^{s}\ell over \mathbb{F}_{2^{m}} .
The proofs of theorems for the rest of cases are similar, and we will give them without proofs.
Theorem 6.7. Let q\equiv 4 \pmod 5 , then one of the following holds.
(1) If f = 2t is even, then
(i) when t is even, there exist (2^{s}+1)^{2e} self-dual cyclic codes of length 5\cdot 2^{s}\ell over \mathbb{F}_{2^{m}} , which are given by
\left((x-1)^{2^{s-1}}C_{\ell}(x)^{2^{s-1}}C_{2\ell}(x)^{2^{s-1}}\prod\limits_{k = 0}^{e-1}C_{\mu^{k}}(x)^{\tau_{k}}C_{-\mu^{k}}(x)^{2^{s}-\tau_{k}}C_{2g^{k}}(x)^{\rho_{k}}C_{-2g^{k}}(x)^{2^{s}-\rho_{k}}C_{5g^{k}}(x)^{2^{s-1}}\right), |
where 0\leq \tau_{k}, \rho_{k}\leq 2^{s} , for any 0\leq k\leq e-1 .
(ii) when t is odd, there exists only one self-dual cyclic codes of length 5\cdot 2^{s}\ell over \mathbb{F}_{2^{m}} , which is given by
\left((x-1)^{2^{s-1}}C_{\ell}(x)^{2^{s-1}}C_{2\ell}(x)^{2^{s-1}}\prod\limits_{k^{'} = 0}^{2e-1}C_{\mu^{k^{'}}}(x)^{2^{s-1}}C_{2\mu^{k^{'}}}(x)^{2^{s-1}}\prod\limits_{k = 0}^{e-1}C_{5\mu^{k}}(x)^{2^{s-1}}\right). |
(2) If f is odd, thenthere exist (2^{s}+1)^{3e/2} self-dual cyclic codes of length 5\cdot 2^{s}\ell over \mathbb{F}_{2^{m}} , which are given by
\begin{eqnarray*} &&\left((x-1)^{2^{s-1}}C_{\ell}(x)^{2^{s-1}}C_{2\ell}(x)^{2^{s-1}}\right.\\ &&\times\left.\prod\limits_{k^{'} = 0}^{e/2-1}C_{\mu^{k^{'}}}(x)^{\tau_{k^{'}}}C_{-\mu^{k^{'}}}(x)^{2^{s}-\tau_{k^{'}}}C_{2\mu^{k^{'}}}(x)^{\rho_{k^{'}}}C_{-2\mu^{k^{'}}}(x)^{2^{s}-\rho_{k^{'}}}C_{5\mu^{k^{'}}}(x)^{\iota_{k^{'}}}C_{-5\mu^{k^{'}}}(x)^{2^{s}-\iota_{k^{'}}}\right). \end{eqnarray*} |
Theorem 6.8. Let q\equiv 2 or 3 \pmod 5 , then one of the following holds.
(1) If 4\mid f . Let f = 4t , then
(i) when t is even, there exist (2^{s}+1)^{2e} self-dual cyclic codes of length 5\cdot 2^{s}\ell over \mathbb{F}_{2^{m}} , which are given by
\left((x-1)^{2^{s-1}}C_{\ell}(x)^{2^{s-1}}\prod\limits_{k^{''} = 0}^{2e-1}C_{\mu^{k}}(x)^{\tau_{k^{''}}}C_{-\mu^{k^{''}}}(x)^{2^{s}-\tau_{k^{''}}}\prod\limits_{k = 0}^{e-1}C_{5\mu^{k}}(x)^{2^{s-1}}\right), |
where 0\leq \tau_{k^{''}}\leq 2^{s} , for any 0\leq k^{''}\leq 2e-1 .
(ii) when t is odd, there exists only one self-dual cyclic codes of length 5\cdot 2^{s}\ell over \mathbb{F}_{2^{m}} , which is given by
\left((x-1)^{2^{s-1}}C_{\ell}(x)^{2^{s-1}}\prod\limits_{k^{'} = 0}^{4e-1}C_{\mu^{k^{'}}}(x)^{2^{s-1}}\prod\limits_{k = 0}^{e-1}C_{5\mu^{k}}(x)^{2^{s-1}}\right), |
(2) If 2\mid f but 4\nmid f , then there exist (2^{s}+1)^{e} self-dual cyclic codes of length 5\cdot 2^{s}\ell over \mathbb{F}_{2^{m}} , which are given by
\left((x-1)^{2^{s-1}}C_{\ell}(x)^{2^{s-1}}\prod\limits_{k = 0}^{e-1}C_{\mu^{k}}(x)^{\tau_{k}}C_{-\mu^{k}}(x)^{2^{s}-\tau_{k}}C_{5\mu^{k}}(x)^{2^{s-1}}\right), |
where 0\leq \tau_{k}\leq 2^{s} , for any 0\leq k\leq e-1 .
(3) If f is odd, then there exist (2^{s}+1)^{e} self-dual cyclic codes of length 5\cdot 2^{s}\ell over \mathbb{F}_{2^{m}} , which are given by
\left((x-1)^{2^{s-1}}C_{\ell}(x)^{2^{s-1}}\prod\limits_{k^{'} = 0}^{\dfrac{e}{2}-1}C_{\mu^{k^{'}}}(x)^{\tau_{k^{'}}}C_{-\mu^{k^{'}}}(x)^{2^{s}-\tau_{k^{'}}}C_{5\mu^{k^{'}}}(x)^{\iota_{k^{'}}}C_{-5\mu^{k^{'}}}(x)^{2^{s}-\iota_{k^{'}}}\right), |
where 0\leq \tau_{k^{'}}, \iota_{k^{'}}\leq 2^{s} , for any 0\leq k^{'}\leq \dfrac{e}{2}-1 .
The first author was supported by the Yuyou Team Support Program of North China University of Technology (No. 107051360019XN137/007) and Yujie Talent Project of North China University of Technology(No. 107051360022XN735).
The authors declare no conflict of interest.
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