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The general two-dimensional divisor problems involving Hecke eigenvalues

  • We consider the general two-dimensional divisor problems involving Hecke eigenvalues, and are able to improve the previous results in this direction.

    Citation: Jing Huang, Taiyu Li, Huafeng Liu, Fuxia Xu. The general two-dimensional divisor problems involving Hecke eigenvalues[J]. AIMS Mathematics, 2022, 7(4): 6396-6403. doi: 10.3934/math.2022356

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  • We consider the general two-dimensional divisor problems involving Hecke eigenvalues, and are able to improve the previous results in this direction.



    As usual, let τ(m) be the divisor function. The famous Dirichlet divisor problem on τ(m) has attracted many authors. For example, the best result to date was established by Bourgain and Watt [2], who obtained that

    myτ(m)=ylogy+(2γ1)y+O(y5171648+ε)

    with Euler's constant γ.

    Let 1k<l be fixed integers. Denote by τ(m;k,l) the number of representations of m as m=mk1ml2, where m1,m2 are natural numbers, that is,

    τ(m;k,l)=m=mk1ml21.

    In 1969, Krätzel [13] proved that

    myτ(m;k,l)=ζ(lk)y1k+ζ(kl)y1l+Δ,

    where

    Δ=mk+ly{ψ((yml)1k)+ψ((ymk)1l)}+O(1),

    ζ is the Riemann zeta function, ψ(z)=z[z]12 and [z] denotes the integer part of z. After that, a lot of results have been established in this direction. We refer to Ivić [8, Chapter 14] for details.

    Now we draw attention to the Hecke eigenvalues. Denote by SL2(Z) the full modular group and by Hκ the set of primitive holomorphic cusp forms g(z) of weight κ for SL2(Z), respectively, where κ2 is an even integer. It is known that Hκ is composed of the eigenfunctions of all Hecke operators. And at the cusp , g(z) has the Fourier expansion:

    g(z)=m=1λg(m)m(κ1)/2e2πimz(Im z>0),

    where λg(m) is the m-th normalized Hecke eigenvalue. For prime number p, one has

    λg(p)=αp+βpandαpβp=|αp|=|βp|=1.

    Then define the Hecke L-function L(g,s) attached to g as

    L(g,s)=m=1λg(m)ms=p(1αpps)1(1βpps)1(Re s>1).

    Further, define the Rankin-Selberg L-function as

    L(g×g,s)=p(1ps)2(1α2pps)1(1β2pps)1(Re s>1).

    Therefore, we have

    L(g×g,s)=ζ(2s)m=1λg(m)2ms:=m=1λg×g(m)ms(Re s>1).

    Many scholars have studied λg(m) and λg×g(m) in various ways and established a lot of results (for example, see [3,4,6,9,11,12,14,15,16,17,18,19,20,21,22,23,24,25], etc.). In addition, one may prefer to consider

    λk,lg×g(m)=m=mk1ml2λg×g(m1)λg×g(m2).

    Recently, Huang, Liu and Xu [7] studied the general two-dimensional divisor problems involving Hecke eigenvalues and proved that for any ε>0,

    where C1=L(sym2g,1)L(g×g,lk) and C2=L(sym2g,1)L(g×g,l2). In this paper, we are able to improve the above result by proving the following theorem.

    Theorem 1.1. Let 1k<l be any fixed integers. Then for any ε>0, we have

    Γg×g(y;k,l)={R1y1k+R2y1l+O(y1k420k5l+ε),if  l2k;R1y1k+O(y35k+ε),if  l>2k,

    where

    R1=L(g×g,lk)L(sym2g,1),R2=L(g×g,kl)L(sym2g,1).

    To prove Theorem 1.1, we mainly use the Perron's formula and the individual and averaged subconvexity bounds for the Riemann zeta-function and the symmetric square L-function.

    Firstly, we introduce the symmetric square L-function L(sym2g,s) defined by

    L(sym2g,s):=p(1ps)1(1α2pps)1(1β2pps)1(Re s>1).

    Write s=σ+it. Let ε be a sufficiently small positive constant, whose value is not necessarily the same at each occurrence.

    Lemma 2.1. We have

    L(g×g,s)=ζ(s)L(sym2g,s)(Re s>1). (2.1)

    Proof. We can find this lemma in [7].

    Lemma 2.2. We have, uniformly for T1,

    T1|L(sym2g,s)|2dtT3(1σ)+ε, (2.2)

    and for 12<σ1,|t|1,

    L(sym2g,s)(1+|t|)max{0,54(1σ)}+ε. (2.3)

    Proof. We can obtain the former result (2.2) from the properties of L(sym2g,s) with standard arguments. The latter result (2.3) was proved by Nunes [18].

    Lemma 2.3. We have, uniformly for T1 and 12σ<1,

    T1|ζ(σ+it)|4dtT1+ε, (2.4)

    and for 12<σ1,|t|1,

    ζ(s)(1+|t|)max{0,1342(1σ)}+ε. (2.5)

    Proof. The result (2.4) with σ=12 is the classical result of Ingham. We can find the result (2.4) in [8]. The third result (2.5) was derived by Bourgain [1].

    Note that

    L(g×g,ks)L(g×g,ls)=m=1λk,lg×g(m)ms. (3.1)

    Then from (3.1) and Perron's formula (see [10, Proposition 5.54]), with a similar argument to [8, page 411] we get

    Γg×g(y;k,l)=(2πi)1ξ+iTξiTL(g×g,ks)L(g×g,ls)yssds+O(y1k+εT), (3.2)

    where ξ=1k+ε and T is a parameter to be determined later. Then we shift the integral line of (3.2) to the parallel line Re s=12k. From Gelbart-Jacquet [5], we note that L(sym2g,s) is holomorphic at s=1. Considering the sizes of l and 2k, we see that s=1k and s=1l will be the only possible simple poles in RT:={s=σ+it:12kσξ,t∣≤T} according to (2.1), and the corresponding residues at s=1k and s=1l are

    R1:=L(g×g,lk)L(sym2g,1),R2:=L(g×g,kl)L(sym2g,1),

    respectively.

    In the following argument, we still carry out the discussion by two cases 2kl and 2k<l. In the case 2kl, both s=1k and s=1l are simple poles in RT. Then we derive from Cauchy's residue theorem,

    Γg×g(y;k,l)=(Ress=1k+Ress=1l)L(g×g,ks)L(g×g,ls)yss+O(y1k+εT)+12πi(12k+iT12kiT+ξ+iT12k+iT+12kiTξiT)L(g×g,ks)L(g×g,ls)yssds:=R1y1k+R2y1l+J1+J2+J3+O(y1k+εT). (3.3)

    To estimate J2 and J3, we also need to divide the integral interval into two arcs A1,A2 and draw support from Lemmas 2.2 and 2.3.

    A1:={s=σ+iT:12kσ1l}. Then in this arc we have

    1TA1yσζ(kσ+ikt)L(sym2g,kσ+ikt)ζ(lσ+ilt)L(sym2g,lσ+ilt)dtmax12kσ1lyσT(1342+54)(1kσ)+(1342+54)(1lσ)T1+εmax12kσ1lT8942+ε(yT13184(k+l))σy12kT225168131l168k+ε+y1lT4784131k84l+ε. (3.4)

    A2:={s=σ+iT:1l<σ1k}. Then in this arc we can get

    1TA2yσζ(kσ+ikt)L(sym2g,kσ+ikt)ζ(lσ+ilt)L(sym2g,lσ+ilt)dσmax1l<σ1kyσT(1342+54)(1kσ)T1+εmax1l<σ1kT4784(yT13184k)σy1kT1+ε+y1lT4784131k84l+ε. (3.5)

    From (3.4) and (3.5) we get

    |J2+J3|T1ξ12kyσζ(kσ+ikt)L(sym2g,kσ+ikt)ζ(lσ+ilt)L(sym2g,lσ+ilt)dσ=T1A1A2yσζ(kσ+ikt)L(sym2g,kσ+ikt)ζ(lσ+ilt)L(sym2g,lσ+ilt)dσy12kT225168131l168k+ε+y1lT4784131k84l+ε+y1k+εT1+ε. (3.6)

    While for J1, we have

    |J1|y12kT1|ζ(12+ikt)L(sym2g,12+ikt)ζ(l2k+ilt)L(sym2g,l2k+ilt)|×t1dt+y12k+εy12klogTmax1T1TT11T1T12|ζ(12+ikt)L(sym2g,12+ikt)ζ(l2k+ilt)×L(sym2g,l2k+ilt)|dt+y12k+ε.

    Then by Hölder's inequality, Lemmas 2.2 and 2.3, one can get

    |J1|y12klogTmaxT1TT11T54(1l2k)+ε1T1T12|ζ(12+ikt)L(sym2g,12+ikt)ζ(l2k+ilt)|dt+y12k+εy12k+ε+y12klogTmaxT1TT54(1l2k)1+ε1(T1T12|ζ(12+ikt)|4dt)14 ×(T1T12|L(sym2g,12+ikt)|2dt)12(T1T12|ζ(l2k+ikt)|4dt)14y12kmaxT1TT325l8k+ε1y12kT325l8k+ε. (3.7)

    Therefore, from (3.3), (3.6) and (3.7), we can establish

    Γg×g(y;k,l)=R1y1k+R2y1l+O(y12kT325l8k+ε+y1lT4784131k84k+ε+y1kT1+ε). (3.8)

    Taking T=y420k5l in (3.8), we have

    Γg×g(y;k,l)=R1y1k+R2y1l+O(y1k420k5l+ε).

    Thus, we prove the first part of Theorem 1.1.

    For 2k<l, s=1k is the only simple pole in the range RT by nothing 1l<12k. Then from Cauchy's residue theorem, we can derive

    Γg×g(y;k,l)=Ress=1kL(g×g,ks)L(g×g,ls)yss+O(y1k+εT)+12πi(12k+iT12kiT+ξ+iT12k+iT+12kiTξiT)L(g×g,ks)L(g×g,ls)yssds:=R1y1k+J1+J2+J3+O(y1k+εT). (3.9)

    To estimate J2 and J3, we also split the integral interval into two arcs A1,A2 but with different ranges from the case 2kl. By a similar argument, we can get

    |J2+J3|y12kT37168+ε+y1k+εT1+ε.

    Note that l2k>1. The estimate of J1 becomes

    |J1|y12kT1|ζ(12+ikt)L(sym2g,12+ikt)ζ(l2k+ilt)L(sym2g,l2k+ilt)|t1dt+y12k+εy12klogTmax1T1TT11T1T12|ζ(12+ikt)L(sym2g,12+ikt)|dt+y12k+εy12klogTmaxT1TT11(T1T12|ζ(12+ikt)|4dt)14(T1T12|L(sym2g,12+ikt)|2dt)12 ×(T1T121dt)14+y12k+εy12kT14+ε.

    Therefore, recalling (3.9) we can get

    Γg×g(y;k,l)=R1y1k+O((y12kT14+y1kT1)Tε). (3.10)

    Taking T=y25k in (3.10), we have

    Γg×g(y;k,l)=R1y1k+O(y35k+ε).

    Thus, the prove of Theorem 1.1 is finished.

    In this paper, we investigate the average behaviors of the Fourier coefficients λk,lg×g(m) and improve the previous estimates in this direction. Here, the condition 1k<l in Theorem 1.1 removes the complexity of discussing the sizes between k and l due to the symmetry. To give a sharper upper bounds for the sum myλk,lg×g(m), we apply some analytic instruments such as Perron's formula, the decomposition of the Rankin-Selberg L-function, and the individual and averaged subconvexity bounds for the Riemann zeta-function and the symmetric square L-function. With the help of results in Theorem 1.1, we can understand the Fourier coefficients λk,lg×g(m) on average more precisely.

    This work was supported by National Natural Science Foundations of China (Grant Nos. 11801328, 11771256 and 11801318) and Young Scholars Program of Shandong University, Weihai (Grant No. 20820211012).

    The authors declare no conflicts of interest.



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