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The Pontryagin dual of the multiplicative group of positive rational numbers

  • The Pontryagin dual of the multiplicative group of positive rational numbers is constructed. Then we study its topological generators and representations.

    Citation: Chaochao Sun, Yuanbo Liu. The Pontryagin dual of the multiplicative group of positive rational numbers[J]. AIMS Mathematics, 2023, 8(9): 20351-20360. doi: 10.3934/math.20231037

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  • The Pontryagin dual of the multiplicative group of positive rational numbers is constructed. Then we study its topological generators and representations.



    We are inspired by Alain Connes' pioneering and excellent work [1], where he realized just those imaginary parts of zeros lying on Re(s)=12 of L-function to be the spectrums of the differential operator on a Hilbert space. Meanwhile, in loc.cit., he proposed a global trace formula in noncommutative geometry, which is equivalent to the Riemann hypothesis of the L-function L(χ,s). Nevertheless the trace formula may be hard to prove.

    When we try to understand his work, we find a key thought is to deal with the action of the group R×+, i.e., the positive real numbers. Since the positive rational number group Q×+ is dense in R×+, the action of R×+ can be reflected by Q×+. On the other hand, Q×+ppZ is the expression of the fundamental theorem of arithmetic by group language. Hence, it is an important object for both group theory and arithmetic. The Pontryagin dual ˆQ×+ of Q×+ is a very interesting space and the Hilbert function space on ˆQ×+ can give some information about L-function.

    First, we calculate the Pontryagin dual group of the multiplicative group of positive rational numbers. Then we get some topological generators in the dual group. Next, we consider the Hilbert space of the dual group, in which we find a function attached to the Dirichlet L-function. For simplicity, we only consider the rational field. The idea can be generalized for general number fields, but it may be more complicated.

    Denote R×+ the multiplicative group of positive real numbers and Q×+ the positive rational numbers. Then Q×+ is dense in R×+ under the usual topology. Since Q×+ppZ, where pZ={pn: nZ}, the set {p: p is prime} is the set of topological generators of R×+.

    The subgroup pZ is discrete in R×+. This fact can be obtained from the topological isomorphism

    log: R×+R,  xlogx.

    Since log(pZ)=Zlogp, we have R+/pZR/Zlogp.

    Let p,q be different primes. Then the group p,q:={pmqn: m,nZ} is dense in R×+. This can be deduced by the following proposition.

    Proposition 2.1. Let a,bR with b0, abQ. Then Za+Zb={ma+nb: m,nZ} is dense in R.

    Proof. Consider the map

    f:Za+ZbR0,  x|x|,

    where R0 is the set of the nonnegative real numbers. If f(Za+Zb){0} has a least absolute value, then Za+Zb is discrete in R. But any discrete subgroup of R is of the form Zx, xR. Since abQ, the free abelian group Za+Zb is of rank 2. Hence, Za+ZbZx for any xR. This means Za+Zb is dense in R.

    Corollary 2.2. Let p,q be different primes. Then the group p,q is dense in R×+.

    Proof. Under the isomorphism log: R×+R, we have

    logp,qZlogp+Zlogq.

    Since logp/logqQ, we have that Zlogp+Zlogq is dense in R by Proposition 2.1. Hence p,q is dense in R×+.

    Corollary 2.3. The cyclic group e2πiθ={e2πinθ: nZ} is dense in S1:={zC: |z|=1} for those θQ.

    Proof. Consider the homomorphism

    fθ: RS1,  xe2πiθx.

    Then the kernel of fθ is Z1θ. Since θQ, the group Z+Z1θ is dense in R by Proposition 2.1. Hence e2πiθ is dense in S1.

    Remark 2.4 (Kronecker's approximation theorem). There is a general form of Kronecker's approximation theorem, that is, the real numbers 1,a1,,an are linearly independent over Q if and only if the set {(ma1,,man): mZ} is dense in the torus Rn/Zn. One can see [2] for details or see Corollary 3.4.

    Let G be a topological group. We denote by ˆG the Pontryagin dual of G. The topological group R×+ is topologically isomorphic to R. Since R is self-dual, so is R×+. The group pZ is the discrete topology. Under the commutative diagram

    we have ^pZR×+/pZ. Further, the dual of Q×+ with the discrete topology is the compact group pR×+/pZ. This can be obtained by the following well known theorem (see [3, Theorem 23.21], which is also a special case of [4, Proposition 1.1.13])

    Theorem 2.5. Let {Gi}i be a set of compact groups. Then

    ^iGiiˆGi.

    Corollary 2.6. Let Q×+ be given by the discrete topology. Then we have

    ^Q×+pR×+/pZpR/ZlogppR/Z1logp.

    Proof. Since ^pZR×+/pZ, we have that ^R×+/pZpZ by the property of Pontryagin dual. Moreover,

    ^pR×+/pZp^R×+/pZppZQ×+.

    Hence, we have ^Q×+pR×+/pZ. The second isomorphism in this corollary is deduced locally by the isomorphisms

    R×+/pZlogR/Zlogp.

    The third isomorphism in this corollary is deduced locally by the following isomorphism restricting on the group Zlogp

    RR, xx(logp)2.

    The inverse limit of Z/mZ with respect to the natural maps is ˆZ:=limmZ/mZ=pZp, where Zp is the p-adic integers. We know that Z is dense in ˆZ and a topological generator is 1 in ˆZ.

    There is a canonical homomorphism from Z to R/Zlogp defined by

    τ: ZR/ZlogpS1,  me2πim/logp.

    Then Z is dense in R/Zlogp by Corollary 2.3, this is because logpQ. Similarly, for the map

    τ: ZR/Z1logpS1,  me2πimlogp,

    Z is dense in R/Z1logp. In general, we have the following theorem.

    Theorem 3.1. For rQ{0}, let τr be the canonical homomorphism

    τr: ZpR/Z1logp,  m(rm)p,

    where (rm)p means that the p-th component is rmR/Z1logp. Then τr(Z) is dense in pR/Z1logp. Hence, any nonzero rational number r is a topological generator of pR/Z1logp.

    Proof. It is easy to see that τr is injective. Let V be the closure of τr(Z) in pR/Z1logp. If VpR/Z1logp, then pR/Z1logpV is a nonempty open set. Let J be a finite set of primes such that for each pJ, there is a nonempty open set Up satisfying (pJUp×pJR/Z1logp)V=. Consider the group pJR/Z1logp and its open set pJUp. Let VJ be the closure of τr(Z) in pJR/Z1logp. Then we have VJpJUp=. This means that τr(Z) is not dense in pJR/Z1logp, which is a contradiction by the following claim.

    Claim: τr(Z) is dense in pJR/Z1logp for each finite set J of primes.

    Consider the isomorphism

    f:RnRn,  (a1,,an)(a1logp1,,anlogpn).

    Then we have f(ni=1Z1logpi)=ni=1Z. Also, f(1,,1)=(logp1,,logpn). By Kronecker's approximation theorem(see Remark 2.4), we just need to show that the numbers 1,logp1,,logpn are linearly independent over Q. Suppose that there is

    a0+a1logp1++anlogpn=0,

    where aiQ. Multiplying the above equation by a suitable integer, we can assume aiZ. Then we have

    log(pa11pann)=a0.

    Taking exponential function on both sides, we have

    pa11pann=ea0.

    Since we are assuming that a0Z, ea0 is rational only when a0=0. Then by the fundamental theorem of arithmetic, we have a1==an=0, that is, the real numbers 1,logp1,,logpn are linearly independent over Q. Hence, τr(Z) is dense in pJR/Z1logp for each finite set J of primes.

    This theorem shows that the group pR/Z1logp is similar to the group pZp. Both have a topological generator, but their topologies are very different. pR/Z1logp is a torus with rank , which is a compact connected Hausdorff space, while pZp is a compact totally disconnected Hausdorff space.

    Corollary 3.2. A topological generator of pR/Zlogp is (,(logp)2,).

    Proof. This follows from Theorem 3.1 and the isomorphism

    pR/Z1logppR/Zlogp,  (xp)p(xp(logp)2)p.

    The following theorem is very useful.

    Theorem 3.3. Let G be a compact group and ˆG be its character group. Then gG is a topological generator if and only if χ(g)1 for each nontrivial character χˆG.

    Proof. See [5, Page 66,Paragraph 3], or [6, Theorem Ⅰ].

    In fact, the above result is equivalent to Kronecker's approximation theorem in the torus case.

    Corollary 3.4. The real numbers 1,a1,,an are linearly independent over Q if and only if the set {(ma1,,man): mZ} is dense in the torus Rn/Zn.

    Proof. The character group of Rn/Zn is Zn. Let χ^Rn/Zn. Then χ can be expressed by

    χ(t1,,tn)=nj=1e2πimjtj=e2πinj=1mjtj,

    where mjZ,tjR/Z. We can view this χ as the vector mχ=(m1,,mn). Then by Theorem 3.3, (a1,,an) is a topological generator of Rn/Zn if and only if

    χ(a1,,an)=e2πinj=1mjaj1

    for every nontrivial character χ.

    Let (a1,,an) be a topological generator of Rn/Zn. Suppose m0+nj=1mjaj=0, where mjQ. Moreover, we can assume mjZ. If m1==mn=0 does not hold, then we get a nontrivial character mχ=(m1,,mn). For this nontrivial character χ, we have χ(a1,,an)=1, which is a contradiction. Hence, m1==mn=0, implying that m0=0. This means 1,a1,,an are linearly independent over Q.

    Let 1,a1,,an be linearly independent over Q. If (a1,,an) is not a topological generator of Rn/Zn, then there exists a nontrivial character χ, i.e., mχ=(m1,,mn)0 such that

    χ(a1,,an)=e2πinj=1mjaj=1.

    Then we have nj=1mjajZ. Hence there exists some mZ such that m+nj=1mjaj=0. Since m1,,mn are not all 0, this means 1,a1,,an are not linearly independent over Q, which is a contradiction. Therefore, (a1,,an) is a topological generator of Rn/Zn.

    Lemma 3.5. Let J be a finite set of primes, kpN, and npZ such that np=0 or (p,np)=1. If

    pJnp/pkp=n

    for some nZ, then we have np=0 for all pJ.

    Proof. Consider the equation

    1=e2πipJnp/pkp=pJe2πinp/pkp.

    Take qJ and let

    mq=pJ,pqpkp.

    Then we have

    1mq=e2πinqmq/qkq.

    Since (mq,q)=1, we have nq=0 for all qJ.

    Remark 3.6. Another proof of Lemma 3.5 suggested by one of reviewers is as following: Multiply pJpkp on the equation pJnp/pkp=n. Then one has pJnpmp=npJpkp, where mp=1pkppJpkp. Modulo p for this equation, one has npmp0modp. Since (mp,p)=1, we have np=0 for all pJ by the condition on np.

    Denote

    Tp=R/Z1logp.

    Then we have the following corollary.

    Corollary 3.7. Consider the compact group

    pTp×Zp.

    Then the image of the diagonal map

    ZpTp×Zp,  n(,n,n,)

    is dense in pTp×Zp.

    Proof. The character group of Tp (resp. Zp) is Z1logp (resp. Z(p), see [3,(25.2)], which is isomorphic to Qp/Zp. Here Qp is the p-adic field). A character χ1 of Tp can be expressed by χ1(t)=e2πimtlogp, where mZ,tTp. However, a character χ2 of Zp can be expressed by χ2(t)=e2πim˜t/pk, where mZ,kN,tZp, ˜tZ such that ˜ttmodpk, (m,p)=1 or m=0. Hence, by Theorem 2.5, any character χ of pTp×Zp can be written as follows

    χ((tp)p,(sp)p)=pJe2πimptplogpe2πinp˜sp/pkp,

    where J is some finite set of primes, mp,npZ,kpN,tpTp,spZp,˜spZ such that ˜spspmodpkp.

    Supposing

    1=χ((1,1))=e2πi(pJmplogp+pJnp/pkp),

    we obtain

    pJmplogp+pJnp/pkpZ.

    So, for some nZ, there is

    log(pJpmp)=npJnp/pkp,

    that is,

    pJpmp=enpJnp/pkp. (3.1)

    Since e is a transcendental number, the right side of the above equation is a rational number only if

    npJnp/pkp=0.

    From Lemma 3.5, we have np=0 for all pJ.

    Furthermore, by (3.1), we obtain

    pJpmp=1.

    The fundamental theorem of arithmetic implies that mp=0, pJ.

    This indicates that χ is a trivial character. Hence, by Theorem 3.3, the image of Z is dense in pTp×Zp.

    Let G be a compact abelian Lie group. All the irreducible representations of G are of dimension 1. Let L2(G) be the Hilbert space of G, which consists of square integrable complex functions. Let ˆG denote the character group of G. Each function fL2(G) can be uniquely expressed by the series

    f=φˆGcφφ,

    where cφC (see [7,§17.B]).

    For G=R/Z, the character group ˆGZ, because the characters φn: GS1 are of the form

    φn: R/ZS1,  θe2πinθ.

    For G=R/Zlogp, the characters of G are

    φnp,p: R/ZlogpS1,  θe2πinpθlogp. (4.1)

    Hence, for G=pR/Zlogp, the character of G is of the form

    φ: pR/ZlogpS1,  (θp)ppe2πinpθplogp=e2πipnpθplogp, (4.2)

    where np=0 for almost all p by Theorem 2.5. The character of pR/Zlogp is φ as in Eq (4.2). All the characters {φ} form a complete orthogonal base of L2(pR/Zlogp). Then for each integrable function fL2(pR/Zlogp), we obtain its Fourier series (see [8, §17])

    f((θp)p)=φˆGcφe2πipnp,φθplogp (4.3)

    where G=pR/Zlogp, cφ=Gf(g)e2πipnp,φθplogpdg, the Haar measure dg of G such that Gdg=1. An integrable function fL2(pR/Zlogp) is uniquely determined by its Fourier series. Furthermore, for each fL2(pR/Zlogp), we have the Parseval equation (see [8, §17])

    G|f(g)|2dg=φˆG|cφ|2,

    where G=pR/Zlogp.

    Take a Dirichlet character χ: (Z/mZ)×S1. The L-function of χ is

    L(χ,s)=n=1χ(n)ns=p11χ(p)ps,

    which is absolutely convergent on Re(s)>1. For p dividing m, we set χ(p)=0. So, taking the logarithm, we have

    logL(χ,s)=plog(1χ(p)ps)=pn=1χ(pn)npns,Re(s)>1, (4.4)

    where we take a branch of the logarithm such that log(R×+)=R.

    From (4.4), we have

    L(χ,s)L(χ,s)=pn=1χ(pn)logppns,Re(s)>1.

    Theorem 4.1. For each sC with Re(s)>12, the value logL(χ,s) corresponds to a function

    fs,χ(θp)p=pn=1χ(pn)logpnpnse2πinθplogplogpL2(pR/Zlogp).

    Furthermore, fs,χ(0)=logL(χ,s) for Re(s)>1.

    Proof. Consider the Eq (4.3). For each character φn,p in Eq (4.1), which can be seen as the character of pR/Zlogp through the projection pR/ZlogpR/Zlogp, we take cp,n=χ(pn)logpnpns; for the other remaining character φ of pR/Zlogp, we let cφ=0. Then we have

    pn=1χ(pn)logpnpns¯(χ(pn)logpnpns)pn=1logpp2nRe(s)=ζ(2Re(s))ζ(2Re(s)),

    which is absolutely convergent when Re(s)>1/2. Hence, we have an integrable function

    fs,χ=pn=1χ(pn)logpnpnse2πinθplogplogpL2(pR/Zlogp).

    It is easy to see fs,χ(0)=logL(χ,s) for Re(s)>1.

    We calculate the Pontryagin dual of the multiplicative group of positive rational numbers under the discrete topology. We find some canonical topological generators in the dual group. Besides, we find some canonical generator in other compact group. We study the representation of the dual group the multiplicative group of positive rational numbers. Furthermore, in the Hilbert function space of the dual group we find a function attached to the Dirichlet L-function, which may be a new way to understand the Dirichlet L-function.

    The authors declare they have not used Artificial Intelligence (AI) tools in the creation of this article.

    We would like to thank Xin Zhang for discussing the first version of the manuscript. We also appreciate the referee's comments to improve the previous version.

    The authors declare that there are no conflicts of interest regarding the publication of this paper.



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