Let λf(n) be the n-th normalized Fourier coefficient of f, which is a primitive holomorphic cusp form of even integral weight k≥2 for the full modular group SL2(Z). Let also σ(n) and ϕ(n) be the sum-of-divisors function and the Euler totient function, respectively. In this paper, we are able to establish the asymptotic formula of the sum of the hybrid arithmetic function λlf(n)σc(n)ϕd(n) over the sparse sequence {n:n=a2+b2}, namely, ∑n≤xλlf(n)σc(n)ϕd(n)r2(n) for 1≤l≤8, where x is a sufficiently large real number, the function r2(n) denotes the number of representations of n as n=a2+b2, a,b,l∈Z and c,d∈R.
Citation: Huafeng Liu, Rui Liu. The sum of a hybrid arithmetic function over a sparse sequence[J]. AIMS Mathematics, 2024, 9(2): 4830-4843. doi: 10.3934/math.2024234
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Let λf(n) be the n-th normalized Fourier coefficient of f, which is a primitive holomorphic cusp form of even integral weight k≥2 for the full modular group SL2(Z). Let also σ(n) and ϕ(n) be the sum-of-divisors function and the Euler totient function, respectively. In this paper, we are able to establish the asymptotic formula of the sum of the hybrid arithmetic function λlf(n)σc(n)ϕd(n) over the sparse sequence {n:n=a2+b2}, namely, ∑n≤xλlf(n)σc(n)ϕd(n)r2(n) for 1≤l≤8, where x is a sufficiently large real number, the function r2(n) denotes the number of representations of n as n=a2+b2, a,b,l∈Z and c,d∈R.
Let H∗k be the set of all normalized primitive holomorphic cusp form of even integral weight k≥2 for the full modular group SL2(Z). The primitive holomorphic cusp form f∈H∗k at the cusp z=∞ has the Fourier expansion
f(z)=∞∑n=1nk−12λf(n)e2πinz, Im(z)>0, |
where λf(n) is the n-th normalized Fourier coefficient. λf(n) is real-valued and has the following multiplicative property
λf(m)λf(n)=∑d|(m,n)λf(mnd2) |
with m,n∈N+. In number theory, the study of the Fourier coefficient λf(n) is of great significance and has attracted attention of many mathematicians. Let d(n) be the Dirichlet divisor function. In 1974, Deligne [1] proved the Ramanujan-Petersson conjecture
|λf(n)|≤d(n). |
In 1927, Hecke [2] established that
∑n≤xλf(n)≪x12. |
Subsequently, Hecke's result was refined by many scholars and the best result now is
∑n≤xλf(n)≪x13logρx, |
where ρ=−0.118⋯, proved by Wu [3]. In 1930, by their powerful method Rankin [4] and Selberg [5] proved
∑n≤xλ2f(n)=cx+O(x35), |
where c is a positive constant depending on f. Recently, the exponent 35 was improved to 35−Δ with Δ≤1560 by Huang [6]. There is also a long history on higher powers sums ∑n≤xλlf(n) with l≥3, and here we refer to the references [7,8] and the references therein for detailed historical descriptions.
Let a,b,l∈Z. Let also r2(n) denote the number of representations of n as n=a2+b2, i.e.,
r2(n)=♯{n=a2+b2,(a,b)∈Z2}. | (1.1) |
In 2013, Zhai [9] studied a related power sum over a sum of two squares and established the following asymptotic formula
∑n≤xλlf(n)r2(n)=xPl(logx)+Of,ε(xθl+ε), |
where P2(t),P4(t),P6(t),P8(t) are polynomials of degree 0,1,4,13, respectively,
Pl(t)≡0 |
for l=3,5,7, and
θ2=811, θ3=1720, θ4=4346, θ5=8386, θ6=184187, θ7=355358, θ8=752755. |
Later, Xu [8] refined and generalized the results of Zhai [9]. Recently, Liu [10] further improved Zhai and Xu's results.
Let σ(n) and ϕ(n) be the sum-of-divisors function and the Euler totient function, respectively. In 2015, Manski et al. [11] proved that
∑n≤xda′(n)σb′(n)ϕc′(n)=xb′+c′+1P2a′−1(logx)+O(xb′+c′+ra′+ε), |
where a′,b′,c′∈R, 2a′∈N, b′+c′>−ra′, Pm(t) is a polynomial in t of degree m and ra′ takes specific values as in [11, (2)].
Let c,d∈R. Many scholars also studied the mean values of the arithmetic function λlf(n)σc(n)ϕd(n) and we refer to [12] for historical results. In detail, Wei and Lao [12] proved that
∑n≤xλlf(n)σc(n)ϕd(n)=xc+d+1Pl(logx)+O(xc+d+θl+ε), |
where P2(t), P4(t), P6(t), P8(t) are polynomials in t of degree 0, 1, 4, 13, respectively, P7(t)≡0, and
θ2=2337, θ4=257299, θ6=201208, θ7=6768, θ8=117118. |
In this paper, motivated by the above results we study the asymptotic behavior of the hybrid arithmetic function λlf(n)σc(n)ϕd(n) over the sparse sequence {n:n=a2+b2}. Define
Sl(f;x)=∑n≤xλlf(n)σc(n)ϕd(n)r2(n), | (1.2) |
where 1≤l≤8, x is a sufficiently large real number, a,b,l∈Z and c,d∈R. By combining some analytic methods with properties of some primitive automorphic L-functions we establish the following theorem.
Theorem 1.1. Let f∈H∗k and λf(n) be the n-th normalized Fourier coefficient of f. Under the notations above, for any ε>0, we have
Sl(f;x)=xc+d+1Al(logx)+Of,ε(xc+d+θl+ε), | (1.3) |
where
Al(t)≡0 |
for l=1,3,5,7, A2(t), A4(t), A6(t), A8(t) are polynomials in t of degree 0, 1, 4, 13, respectively, and
θ1=12=0.5,θ2=1217=0.7058⋯,θ3=1720=0.85,θ4=52095629=0.9254⋯,θ5=8386=0.9651⋯,θ6=64876607=0.9818⋯,θ7=353356=0.9915⋯,θ8=4885749067=0.9957⋯. |
For a random variable X defined on a countable sample space V, let E(X) denote the mathematical expectation of X. With the help of Theorem 1.3, we can obtain the asymptotic mathematical expectation, denoted by E(λlf(n)σc(n)ϕd(n)r2(n))1≤n≤x, of λlf(n)σc(n)ϕd(n) over the sample space
1≤n≤x, n=a2+b2. |
Theorem 1.2. Under the same notations as in Theorem 1.3, we have
E(λlf(n)σc(n)ϕd(n)r2(n))1≤n≤x=π−1xc+dAl(logx)+Of,ε(xc+d+θl−1+ε). |
In the following Section 2, we give some preliminary lemmas. In Sections 3 and 4, we complete the proofs of Theorems 1.1 and 1.2, respectively.
Notation. Throughout this paper, we apply the letter ε to represent a sufficiently small positive constant, whose value may change from statement to statement. The constants, both explicit and implicit, in Vinogradov symbols may depend on ε and f.
We first introduce some L-functions and then give some necessary lemmas. As usual, we define Riemann zeta function ζ(s) and Dirichlet L-function L(s,χ) as
ζ(s)=∞∑n=11nsandL(s,χ)=∞∑n=1χ(n)ns | (2.1) |
for Re(s)>1, respectively. For the n-th normalized Fourier coefficient λf(n), Deligne [1] showed that for any prime p there are two complex numbers αf(p) and βf(p) satisfying
λf(p)=αf(p)+βf(p), |αf(p)|=|βf(p)|=αf(p)βf(p)=1. | (2.2) |
Thus, the Hecke L-function associated to f∈H∗k can be represented as
L(s,f)=∞∑n=1λf(n)ns=∏p(1−αf(p)p−s)−1(1−βf(p)p−s)−1, Re(s)>1. | (2.3) |
Then, the j-th symmetric power L-function with f∈H∗k can be defined as, for Re(s)>1,
L(s,symjf):=∏pj∏m=0(1−αj−mf(p)βmf(p)p−s)−1=∞∑n=1λsymjf(n)ns. | (2.4) |
Note that
L(s,sym0f)=ζ(s) |
and
L(s,sym1f)=L(s,f). |
For Re(s)>1, the j-th symmetric power L-function twisted by the Dirichlet character χ is defined as
L(s,symjf×χ):=∏pj∏m=0(1−αj−mf(p)βmf(p)χ(p)p−s)−1=∞∑n=1λsymjf(n)χ(n)ns. | (2.5) |
Recall (1.1). It was showed by Iwaniec [13] that
r2(n)=4∑d|nχ(d), |
where χ(d) is the non-trivial Dirichlet character modulo 4. Let r(n) denote r2(n)/4. Since χ(n) is completely multiplicative, one has
r(p)=∑d|pχ(d)=1+χ(p). |
Therefore, we can write
Sl(f;x)=∑n≤xλlf(n)σc(n)ϕd(n)r2(n)=4∑n≤xλlf(n)σc(n)ϕd(n)r(n). |
Now, we turn to give some necessary lemmas. From the recent deep results of Newton and Thorne [14,15], we know that all symjf with j∈N+ are automorphic cuspidal representations of GLj+1. That is, the j-th symmetric power L-function with L(s,symjf) with j∈N+ has analytic continuation as an entire function in the whole plane and certain functional equations. Thus, L(s,symjf) with j∈N+ are general L-functions in sense of Perelli [16].
Lemma 2.1. For any
ε>0, 12≤σ≤1and|t|≥1, |
we have
ζ(σ+it)≪(1+|t|)1342(1−σ)+ε,L(σ+it,f)≪(1+|t|)23(1−σ)+ε,L(σ+it,sym2f)≪(1+|t|)65(1−σ)+ε,L(σ+it,symjf)≪(1+|t|)j+12(1−σ)+ε, j=3,4,5,⋯. |
Proof. The former three results can be found in the works [17, Theorem 5], [9, Lemma 2.3] and [18, Corollary 1.2], respectively. The last result follows from [16, Theorem 4] and [19, Proposition 1] plainly.
Lemma 2.2. Let χ be the non-trivial Dirichlet character modulo 4. For any
ε>0, 12≤σ≤1and|t|≥1, |
one has
L(σ+it,χ)≪(1+|t|)1342(1−σ)+ε,L(σ+it,f×χ)≪(1+|t|)23(1−σ)+ε,L(σ+it,sym2f×χ)≪(1+|t|)65(1−σ)+ε,L(σ+it,symjf×χ)≪(1+|t|)j+12(1−σ)+ε, j=3,4,5,⋯. |
Proof. Since χ is the non-trivial Dirichlet character modulo 4, twisting by the character χ does not affect subconvexity bounds and convexity bounds of L-functions in the t's aspect.
Lemma 2.3. Let f∈H∗k and χ be the non-trivial Dirichlet character modulo 4. Then, for any ε>0, j∈N+ and |t|≥1, we have
∫2TT|L(σ+it,symjf)|2dt≪|T|(j+1)(1−σ)+ε |
and
∫2TT|L(σ+it,symjf×χ)|2dt≪|T|(j+1)(1−σ)+ε. |
Proof. The first result is in [16, Lemma 13]. The second result follows from the first result by the same reason as in Lemma 2.2.
Lemma 2.4. For any U≥U0, where U0 is a sufficiently large constant, there exists T∗∈(U,2U), such that
maxσ≥12|ζ(σ±iT∗)|≤exp(C(loglogU)2), |
where C>0 is an absolute constant.
Proof. This result is proved by Ramachandra and Sankaranarayanan [20, Lemma 2].
Lemma 2.5. For any ε>0, we have
∫T0|ζ(57+it)|12dt≪T1+ε, |
uniformly for T≥1.
Proof. This result was established by Ivić [21, Theorem 8.4 and (8.87)].
Lemma 2.6. Let
F(s):=∑n≥1anns |
be a Dirichlet series with a finite abscissa of absolute convergence σa. Suppose there exists a real number α≥0 such that
(i)
∑n≥1|an|n−σ≪(σ−σa)−α, |
where σa<σ≤σa+1, and that B is a non-decreasing function satisfying
(ii)
|an|≤B(n), |
where n≥1. Then, for
x≥2, T≥2,andσ≤σa, |
κ:=σa−σ+1logx, |
we have
∑n≤xanns=12πi∫κ+iTκ−iTF(s+w)xwwdw+O(xσa−σ(logx)αT+B(2x)xσ(1+xlogTT)). |
Proof. This is the well-known Perron's formula, which can be found in [22, Corollary 2.4].
Lemma 2.7. Let
Fl(s)=∞∑n=1λlf(n)σc(n)ϕd(n)r(n)ns. |
Then, for l=1,⋯,8, we have
Fl(s)=Gl(s−c−d)Hl(s), |
where
G1(s)=L(s,f)L(s,f×χ),G2(s)=ζ(s)L(s,χ)L(s,sym2f)L(s,sym2f×χ),G3(s)=L2(s,f)L2(s,f×χ)L(s,sym3f)L(s,sym3f×χ),G4(s)=ζ2(s)L2(s,χ)L3(s,sym2f)L3(s,sym2f×χ)L(s,sym4f)L(s,sym4f×χ),G5(s)=L5(s,f)L5(s,f×χ)L4(s,sym3f)L4(s,sym3f×χ)L(s,sym5f)×L(s,sym5f×χ),G6(s)=ζ5(s)L5(s,χ)L9(s,sym2f)L9(s,sym2f×χ)L5(s,sym4f)L5(s,sym4f×χ)×L(s,sym6f)L(s,sym6f×χ),G7(s)=L14(s,f)L14(s,f×χ)L14(s,sym3f)L14(s,sym3f×χ)L6(s,sym5f)×L6(s,sym5f×χ)L(s,sym7f)L(s,sym7f×χ),G8(s)=ζ14(s)L14(s,χ)L28(s,sym2f)L28(s,sym2f×χ)L20(s,sym4f)×L20(s,sym4f×χ)L7(s,sym6f)L7(s,sym6f×χ)L(s,sym8f)×L(s,sym8f×χ), |
where χ is the non-trivial Dirichlet character modulo 4 and Hl(s) is absolutely convergent for
Re(s)≥c+d+12. |
Proof. Here, we give the detailed proof for l=7 as an example, since the remaining cases can be proven by following a similar argument.
For l=7, due to the multiplicative property of λf(n), σ(n), ϕ(n) and r(n), we have
F7(s)=∏p∞∑k=0λ7f(pk)σc(pk)ϕd(pk)r(pk)pks=∏p(1+λ7f(p)σc(p)ϕd(p)r(p)ps+λ7f(p2)σc(p2)ϕd(p2)r(p2)p2s+⋯)=∏p(1+(αf(p)+βf(p))7(p+1)c(p−1)dr(p)ps+(α3f(p)−β3f(p)αf(p)−βf(p))7(p2+p+1)c(p2−p)dr2(p)p2s+⋯)=∏p(1+(αf(p)+βf(p))7(1+χ(p))ps−c−d+O(p2(c+d−σ)+pc+d−σ−1)). |
Further, by the binomial theorem, (2.2) and (2.4) we have
F7(s)=∏p(1+(αf(p)+βf(p))7(1+χ(p))p−(s−c−d)+O(p2(c+d−σ)+pc+d−σ−1))=∏p(1+(α7f(p)+7α5f(p)+21α3f(p)+35αf(p)+35βf(p)+21β3f(p)+7β5f(p)+β7f(p))p−(s−c−d)+(α7f(p)+7α5f(p)+21α3f(p)+35αf(p)+35βf(p)+21β3f(p)+7β5f(p)+β7f(p))p−(s−c−d)χ(p)+O(p2(c+d−σ)+pc+d−σ−1))=L(s−c−d,sym7f)L(s−c−d,sym7f×χ)×∏p(1+(6α5f(p)+20α3f(p)+34αf(p)+34βf(p)+20β3f(p)+6β5f(p))×p−(s−c−d)+(6α5f(p)+20α3f(p)+34αf(p)+34βf(p)+20β3f(p)+6β5f(p))×p−(s−c−d)χ(p)+O(p2(c+d−σ)+pc+d+1+σ))=L(s−c−d,sym7f)L(s−c−d,sym7f×χ)L6(s−c−d,sym5f)×L6(s−c−d,sym5f×χ)∏p(1+(14α3f(p)+28αf(p)+28βf(p)+14β3f(p))p−(s−c−d)+(14α3f(p)+28αf(p)+28βf(p)+14β3f(p))p−(s−c−d)×χ(p)+O(p2(c+d−σ)+pc+d−σ−1))=L(s−c−d,sym7f)L(s−c−d,sym7f×χ)L6(s−c−d,sym5f)×L6(s−c−d,sym5f×χ)L14(s−c−d,sym3f)L14(s−c−d,sym3f×χ)×∏p(1+(14αf(p)+14βf(p))p−(s−c−d)+(14αf(p)+14βf(p))p−(s−c−d)×χ(p)+O(p2(c+d−σ)+pc+d−σ−1))=L(s−c−d,sym7f)L(s−c−d,sym7f×χ)L6(s−c−d,sym5f)×L6(s−c−d,sym5f×χ)L14(s−c−d,sym3f)L14(s−c−d,sym3f×χ)×L14(s,f)L14(s,f×χ)∏p(1+O(p2(c+d−σ)+pc+d−σ−1))=G7(s−c−d)H7(s), |
where H7(s) converges absolutely and uniformly for Re(s)>c+d+12.
In this section, we shall give the proof of Theorem 1.1. Here, we shall give the detailed proofs for the cases l=7,8. For the cases l=1,3,5, the proofs are similar to the proof of l=7. For the cases l=2,4,6, the proofs are similar to the proof of l=8.
We first handle the case l=7. Using Lemma 2.6 to ∑n≤xλ7f(n)σc(n)ϕd(n)r(n), we get
∑n≤xλ7f(n)σc(n)ϕd(n)r(n)=12πi∫c+d+1+ε+iTc+d+1+ε−iTG7(s−c−d)H7(s)xssds+O(xc+d+1+εT−1). |
Since, from Lemma 2.7, G7(s−c−d)H7(s)xss has no poles in the range
c+d+12+ε≤σ≤c+d+1+ε |
and |t|≤T, by Cauchy's Residue Theorem we obtain
∑n≤xλ7f(n)σc(n)ϕd(n)r(n)=12πi(∫c+d+1+ε+iTc+d+12+ε+iT+∫c+d+12+ε+iTc+d+12+ε−iT+∫c+d+12+ε−iTc+d+1+ε−iT)G7(s−c−d)H7(s)xssds+O(xc+d+1+εT−1):=12πi(I71+I72+I73)+O(xc+d+1+εT−1). |
For the horizontal segments, since H7(s) is absolutely convergent in Re(s)>c+d+12, by Lemmas 2.1 and 2.2, we have
|I71+I73|≪∫1+ε12+ε|G7(s)xc+d+σT−1|dσ≪xc+d∫1+ε12+ε|G7(s)xσT−1|dσ≪xc+d+εmax12+ε≤σ≤1+εxσT(23×14+42×14+62×6+82)×(1−σ)×2−1≪xc+d+εmax12+ε≤σ≤1+εT3533(xT3563)σ≪xc+d+1+εT−1+xc+d+12+εT1753. | (3.1) |
Then, for the vertical segment, by Lemmas 2.1–2.3 and Cauchy's inequality, we have
|I72|≪∫T1|G7(12+ε+it)xc+d+12+εc+d+12+ε+it|dt≪xc+d+12+ε+xc+d+12+ε∫T1|G7(12+ε+it)1t|dt≪xc+d+12+ε+xc+d+12+εlogTmax1≤T1≤T1T1(maxT12≤t≤T1|L14(12+ε+it,f)×L14(12+ε+it,f×χ)L14(12+ε+it,sym3f)×L14(12+ε+it,sym3f×χ)L6(12+ε+it,sym5f)×L6(12+ε+it,sym5f×χ)|)(∫T1T12|L(12+ε+it,sym7f)|2dt)12×(∫T1T12|L(12+ε+it,sym7f×χ)|2dt)12≪xc+d+12+ε+xc+d+12+εmax1≤T1≤TT−1+(23×14+42×14+62×6)×12×2+8×12×12×21≪xc+d+12+ε+xc+d+12+εT1753≪xc+d+12+εT1753. | (3.2) |
Thus, according to (3.1) and (3.2), we get
∑n≤xλ7f(n)σc(n)ϕd(n)r(n)=O(xc+d+1+εT−1+xc+d+12+εT1753). |
Taking
T=x3356, |
we have
S7(f;x)=O(xc+d+353356+ε). |
Then, we turn to the case l=8. Using Lemma 2.6 to ∑n≤xλ8f(n)σc(n)ϕd(n)r(n), we get
∑n≤xλ8f(n)σc(n)ϕd(n)r(n)=12πi∫c+d+1+ε+iTc+d+1+ε−iTG8(s−c−d)H8(s)xssds+O(xc+d+1+εT−1). |
Since, from Lemma 2.7, G8(s−c−d)H8(s)xss only has one pole at s=c+d+1 of order 14 in the range
c+d+12+ε≤σ≤c+d+1+ε |
and |t|≤T, by Cauchy's Residue Theorem again we obtain
∑n≤xλ8f(n)σc(n)ϕd(n)r(n)=Ress=c+d+1{F8(s)xss}+12πi(∫c+d+1+ε+iTc+d+57+ε+iT+∫c+d+57+ε+iTc+d+57+ε−iT+∫c+d+57+ε−iTc+d+1+ε−iT) G8(s−c−d)H8(s)xssds+O(xc+d+1+εT−1):=xc+d+1A′13(logx)+12πi(I81+I82+I83)+O(xc+d+1+εT−1), |
where A′13(t) is a polynomial in t of degree 13.
For the horizontal segments, since H8(s) is absolutely convergent in
Re(s)>c+d+57, |
by Lemmas 2.1, 2.2 and 2.4, we have
|I81+I83|≪∫1+ε57+ε|G8(s)xc+d+σT−1|dσ≪xc+d∫1+ε57+ε|G8(s)xσT−1|dσ≪xc+d+εmax57+ε≤σ≤1+εxσT1342×(1−σ)×14+(65×28+52×20+72×7+92)×(1−σ)×2−1≪xc+d+εmax57+ε≤σ≤1+εT342815(xT344315)σ≪xc+d+1+εT−1+xc+d+57+εT6781105. | (3.3) |
Then, for the vertical segment, by Lemmas 2.1–2.3 and 2.5, we have
|I82|≪∫T1|G8(57+ε+it)xc+d+57+εc+d+57+ε+it|dt≪xc+d+57+ε+xc+d+57+ε∫T1|G8(57+ε+it)1t|dt≪xc+d+57+ε+xc+d+57+εlogTmax1≤T1≤T1T1(maxT12≤t≤T1|ζ2(57+ε+it)×L14(57+ε+it,χ)L28(57+ε+it,sym2f)×L28(57+ε+it,sym2f×χ)L20(57+ε+it,sym4f)×L20(57+ε+it,sym4f×χ)L7(57+ε+it,sym6f)×L7(57+ε+it,sym6f×χ)L(57+ε+it,sym8f)×L(57+ε+it,sym8f×χ)|)∫T1T12|ζ(57+ε+it)|12dt≪xc+d+57+ε+xc+d+57+εmax1≤T1≤TT−1+1342×27×16+(65×28+52×20+72×7+92)×27×2+11≪xc+d+57+ε+xc+d+57+εT48332735≪xc+d+57+εT48332735. | (3.4) |
Thus, according to (3.3) and (3.4), we get
∑n≤xλ8f(n)σc(n)ϕd(n)r(n)=xc+d+1A′13(logx)+O(xc+d+1+εT−1+xc+d+57+εT48332735). |
Taking
T=x21049067, |
we have
S8(f;x)=xc+d+1A13(logx)+O(xc+d+4885749067+ε). |
For the Gauss circle problem, we have the following famous result
∑n≤xn=a2+b21=πx+O(x13), |
where a,b∈Z. This result can be found in [13, Corollary 4.9]. Then, by the definition of mathematical expectation and Theorem 1.3, for l=1,2,⋯,8, one has
E(λlf(n)σc(n)ϕd(n))1≤n≤xn=a2+b2=∑n≤xn=a2+b2λlf(n)σc(n)ϕd(n)∑n≤xn=a2+b21=xc+d+1Al(logx)+O(xc+d+θl+ε)πx+O(x13)=π−1xc+dAl(logx)+O(xc+d+θl−1+ε), |
where the notations of Al(logx) and θl are the same as ones in Theorem 1.3. Thus, we complete the proof of Theorem 1.2.
In this paper, we establish the asymptotic formula of the sum of the hybrid arithmetic function λlf(n)σc(n)ϕd(n) over the sparse sequence {n:n=a2+b2}, i.e., ∑n≤xλlf(n)σc(n)ϕd(n)r2(n) for 1≤l≤8, where x is a sufficiently large real number, λf(n) is the n-th normalized Fourier coefficient of the primitive holomorphic cusp form f of even integral weight k≥2 for SL2(Z), σ(n) and ϕ(n) are the sum-of-divisors function and the Euler totient function, r2(n) denotes the number of representations of n as n=a2+b2, a,b,l∈Z and c,d∈R. In addition, we also study the mathematical expectation of this hybrid arithmetic function. With the help of Theorems 1.3 and 1.2, we can understand the asymptotic behaviors of the hybrid arithmetic function λlf(n)σc(n)ϕd(n) over the sparse sequence {n:n=a2+b2} more precisely.
The authors declare they have not used Artificial Intelligence (AI) tools in the creation of this article.
The authors would like to thank the referee for many useful comments on the manuscript. This work is supported by the National Natural Science Foundation of China (Grant No. 12171286).
The authors declare that they have no conflicts of interest.
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