Large values of $L$-functions on $1$-line
Anup B. Dixit, Kamalakshya Mahatab

TL;DR
This paper establishes lower bounds for a broad class of $L$-functions on the 1-line, showing they can attain arbitrarily large values, generalizing previous results for the Riemann zeta-function.
Contribution
It extends known lower bound results for the Riemann zeta-function to a wider family of $L$-functions, including Dedekind zeta and Rankin-Selberg $L$-functions.
Findings
Existence of arbitrarily large values of $L$-functions on the 1-line.
Generalization of previous bounds for the Riemann zeta-function.
Lower bounds applicable to Dedekind zeta and Rankin-Selberg $L$-functions.
Abstract
In this paper, we study lower bounds of a general family of -functions on the -line. More precisely, we show that for any in this family, there exists arbitrary large such that , where is the order of the pole of at . This is a generalization of the same result of Aistleitner, Munsch and the second author for the Riemann zeta-function. As a consequence, we get lower bounds for large values of Dedekind zeta-functions and Rankin-Selberg -functions of the type on the -line.
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Taxonomy
TopicsAnalytic Number Theory Research · Advanced Mathematical Identities · Algebraic Geometry and Number Theory
Large values of -functions on -line
Anup B. Dixit
Department of Mathematics and Statistics
Queen’s University
Jeffrey Hall, 48 University Ave
Kingston
Canada, ON
K7L 3N8
and
Kamalakshya Mahatab
Kamalakshya Mahatab, Department of Mathematics and Statistics, University of Helsinki, P. O. Box 68, FIN 00014 Helsinki, Finland
[email protected], k̇[email protected]
Abstract.
In this paper, we study lower bounds of a general family of -functions on the -line. More precisely, we show that for any in this family, there exists arbitrary large such that , where is the order of the pole of at . This is a generalization of the same result of Aistleitner, Munsch and the second author for the Riemann zeta-function. As a consequence, we get lower bounds for large values of Dedekind zeta-functions and Rankin-Selberg -functions of the type on the -line.
Key words and phrases:
Dedekind zeta function, values on 1-line
2010 Mathematics Subject Classification:
11M41
ABD is supported by the Coleman postdoctoral fellowship of Queen’s university
KM is supported by Grant 227768 of the Research Council of Norway and Project 1309940 of Finnish Academy.
1. Introduction
The growth of the Riemann zeta-function in the critical strip has been of interest to number theorists for a long time. In this context, the upper bound is predicted by the Lindelöf hypothesis, which claims that for any and . This is, in fact a consequence of the famous Riemann hypothesis. Although there is significant progress towards this problem, no unconditional proof is known (see [22] for more details).
A more intricate question is to investigate how large can be for a fixed and , some interval. In this direction, Balasubramanian and Ramachandra [7] showed that
[TABLE]
where is a positive constant, and denotes . From now on, we will denote by . This result was improved by Bondarenko and Seip [9] in a larger interval and was later optimized by Bretche and Tenenbaum [10], who showed that
[TABLE]
For and , Aistleitner [1] proved that
[TABLE]
On the other hand, we expect much finer results for large values of -functions on . In [12], Granville and Soundararajan used techniques of diophantine approximation to show that
[TABLE]
for arbitrarily large . This is an improvement on the previous bounds given by Levinson [14]. Granville and Soundararajan [12] conjectured that
[TABLE]
where is an explicitly computable constant.
In 2017, Aistleitner, Munsch and the second author [2] used the resonance method to prove that there is a constant such that
[TABLE]
Note that this result essentially matches (1), however, the size of the interval here is much larger. Unfortunately, over shorter intervals , very little seems to be known regarding large values of (see [5], [6] for further details).
In this paper, we generalize (2) to a large class of -functions, namely , which conjecturally contains the Selberg class . We establish (2) for elements in with non-negative Dirichlet coefficients. The key difference between and is that elements in satisfy a polynomial Euler-product which is a more restrictive condition than that in . However, the functional equation in is replaced by a weaker “growth condition” in . This is a significant generalization because most Euler products, which have an analytic continuation exhibit a growth condition, but perhaps not a functional equation. As applications, we prove the analogue of (2) for Dedekind zeta-functions and Rankin-Selberg -functions given by . We also prove a generalized Merten’s theorem for as a precursor to the proof of our main theorem.
The resonance method with a similar resonator was used by Aistleitner, Munsch, Peyrot and the second author [3] to establish large values of Dirichlet -functions with a given conductor at . Perhaps, a similar method can also be used to establish large values over more general orthogonal families of -functions in .
1.1. The class
In 1989, Selberg [20] introduced a class of -functions , which is expected to encapsulate all naturally occurring -functions arising from arithmetic and geometry.
Definition 1.1** (The Selberg class).**
The Selberg class consists of meromorphic functions satisfying the following properties.
- (i)
Dirichlet series* - It can be expressed as a Dirichlet series*
[TABLE]
which is absolutely convergent in the region . We also normalize the leading coefficient as . 2. (ii)
Analytic continuation* - There exists a non-negative integer , such that is an entire function of finite order.* 3. (iii)
Functional equation* - There exist real numbers and , complex numbers and , with and , such that*
[TABLE]
satisfies the functional equation
[TABLE] 4. (iv)
Euler product* - There is an Euler product of the form*
[TABLE]
where
[TABLE]
with for some . 5. (v)
Ramanujan hypothesis* - For any ,*
[TABLE]
The Euler product implies that the coefficients are multiplicative, i.e., when . Moreover, each Euler factor also has a Dirichlet series representation
[TABLE]
which is absolutely convergent on and non-vanishing on , where is as in .
For the purpose of this paper, we need a stronger Euler-product to ensure that the Euler factors factorize completely and further require a zero free region near -line, similar to what we notice in the proof of prime number theorem. However, we can replace the functional equation with a weaker condition on the growth of -functions on vertical lines. This leads to the definition of the class .
Definition 1.2** (The class ).**
The class consists of meromorphic functions satisfying (i), (ii) as in the above definition and further satisfies
- (a)
Complete Euler product decomposition* - The Euler product in (4) factorizes completely, i.e.,*
[TABLE]
with and . 2. (b)
Zero-free region* - There exists a positive constant , depending on , such that has no zeros in the region*
[TABLE]
except the possible Siegel-zero of . 3. (c)
Growth condition* - For , define*
[TABLE]
Then,
[TABLE]
is bounded for .
One expects to satisfy conditions and . In fact, the Riemann zeta-function, the Dirichlet -functions, the Dedekind zeta-functions and the Rankin-Selberg -functions can be all shown to satisfy conditions and . Furthermore, for elements in the growth condition is a consequence of the functional equation (3). However, it is possible to have -functions not obeying a functional equation to satisfy the growth condition. One can consider linear combination of elements in to see this. A family of -functions based on growth condition was introduced by V. K. Murty in [17] and the reader may refer to [11] for more details on this family. Also the Igusa zeta-function, and the zeta function of groups have Euler products but may not have functional equation, which is discussed in [19].
1.2. The Main Theorem
In this paper, we produce a lower bound for large values of -functions in on the -line. For a meromorphic function having a pole of order at , define
[TABLE]
Theorem 1.3**.**
Let have non-negative Dirichlet coefficients and a pole of order at . Then, there exists a constant depending on such that
[TABLE]
where and is the Euler-Mascheroni constant.
In the above theorem, since , we clearly have . This is important because if has no pole at , it is possible for to grow very slowly on the 1-line.
As an immediate corollary, we get the following result for Dedekind zeta-functions . Let be a number field. The Dedekind zeta-function is defined on as
[TABLE]
where runs over all non-zero integral ideals and runs over all non-zero prime ideals of . The function has an analytic continuation to the complex plane except for a simple pole at . Furthermore, satisfies properties and therefore . Thus, by Theorem 1.3, we have
Corollary 1**.**
For a number field , there exists a constant depending on such that
[TABLE]
where , with being the residue of at .
The -function associated to the Rankin-Selberg convolution of any two holomorphic newforms and , denoted by , is in the Selberg class. Moreover, it can also be shown that . Here and are normalized Hecke eigenforms of weight . It is known that if has a pole at , then . Hence, from Theorem 1.3, we have the following.
Corollary 2**.**
For a normalized Hecke eigenform , there exists a constant such that
[TABLE]
where , with being the residue of at .
The result obtained in Theorem 1.3 is a refined version of the bound established by Aistleitner-Pańkowski [4], which states that if is in the Selberg class and satisfies the prime number theorem, namely,
[TABLE]
then for large ,
[TABLE]
Furthermore, since we are assuming the zero-free region in , using [13, Theorem 1], we have . Hence, we get a slightly more refined result than (8), but on a larger interval .
The poles of any element in the Selberg class are expected to arise from the Riemann zeta-function. More precisely, if has a pole of order at , then is expected to be entire and in . Thus, it is not surprising to expect the lower bound in Theorem 1.3 to be of the order .
It is possible to generalize Theorem 1.3 to the Beurling zeta-function [8] by constructing a suitable resonator over Beurling numbers instead of integers. However, this will carry us far afield from our current focus. Hence, we relegate it to future research.
2. Mertens’ theorem for the class
In 1874, Mertens [15] proved the following estimate for truncated Euler-product of , which is also known as Mertens’ third theorem given by
[TABLE]
where denotes the Euler-Mascheroni constant. The analogue of Mertens’ theorem for number fields was proved by Rosen [18], who showed that
[TABLE]
where denotes the residue of at . The Mertens theorem for the extended Selberg class satisfying conditions and was proved by Yashiro [23] in 2013. Following similar approach, one can establish Mertens’ theorem for , where we replace the functional equation by the growth condition. However, Yashiro’s paper [23] seems to be available only on arXiv. Hence, we include the proof for the sake of completeness.
Theorem 2.1**.**
Let . Suppose that has a pole of order at and be as in (7). Then, for a constant ,
[TABLE]
Proof.
We closely follow the method of Yashiro [23]. Denote by
[TABLE]
Let
[TABLE]
By the complete Euler product (6), we have if and for some . Since
[TABLE]
we have . Write
[TABLE]
It is easy to estimate the second and third term above as follows.
[TABLE]
Also,
[TABLE]
From (2), we get
[TABLE]
Setting and and using Perron’s formula, we get
[TABLE]
Let . Choosing sufficiently large, we can ensure that there are no Siegel zeros for in the region . Hence from the condition , has no zeros in the region and and has a pole of order at .
Consider the contour joining and . By the residue theorem, we have
[TABLE]
We now estimate the above integral. Suppose . By the growth condition , we have
[TABLE]
where . Thus, for our choice of and , we get for
[TABLE]
Hence, we have
[TABLE]
for some . Similarly, we also get
[TABLE]
We use the following result due to Landau (see [16, p. 170, Lemma 6.3]) to esimate the other terms in (10).
Lemma 2.2**.**
Let be an analytic function in the region containing the disc , supposing for and . Fix r and R such that . Then, for we have
[TABLE]
where is a zero of .
Let , and in the above Lemma 2.2. Using the zero-free region , we get
[TABLE]
We now have the estimate
[TABLE]
for some . Similarly, we also have
[TABLE]
Using the estimates (2), (12), (2) and (14) in the Equation (10) and choosing , we get
[TABLE]
Let denote the circle of radius centered at [math]. Then,
[TABLE]
Hence, it suffices to estimate the above integral. Since has a pole of order at ,
[TABLE]
Writing , we get
[TABLE]
The integral on the right hand side is
[TABLE]
By the series expansion of exponential function, we have
[TABLE]
Similarly,
[TABLE]
But the Euler-Mascheroni constant satisfies the identity
[TABLE]
Thus, we have
[TABLE]
Combining the estimates above (16)-(2), we get
[TABLE]
Taking exponential on both sides and using the fact that for , we are done.
∎
3. Proof of the main theorem
For , define
[TABLE]
We use the following approximation lemma.
Lemma 3.1**.**
For large ,
[TABLE]
for and .
Proof.
From the Euler product of , we have for ,
[TABLE]
Let and let be any sufficiently large constant. Define
[TABLE]
Applying Perron’s summation formula as in [21, Theorem II.2.2], we get
[TABLE]
Now, we shift the path of integration to the left. By the zero-free region of , the only pole of the above integrand in and is at . Therefore, we have
[TABLE]
where is the contour joining and . Since, on , we get
[TABLE]
and
[TABLE]
where all implied constants are absolute. Substituting the bounds from (20) and (21) in (19), for , we obtain
[TABLE]
Similarly we may argue when is negative. ∎
By Lemma 3.1, it suffices to show Theorem 1.3 for . We closely follow the argument in [2]. Set
[TABLE]
and for primes set
[TABLE]
Also set and for . Extend the definition completely multiplicatively to define for all integers . Now define
[TABLE]
Then we have
[TABLE]
where is the prime counting function and is the first Chebyshev function. By partial summation, we know that
[TABLE]
By our choice of , we get
[TABLE]
From the Euler product, has the following series representation
[TABLE]
and hence we get
[TABLE]
We have
[TABLE]
Since , we get
[TABLE]
Set and recall that its Fourier transform is positive. Using (22), we have
[TABLE]
and
[TABLE]
Using the fact that and the positivity of the Fourier coefficients of , we also have the following lower bound
[TABLE]
By a similar argument, again using the positivity of the Fourier coefficients, we have
[TABLE]
So, we restrict ourselves to primes in the truncated Euler-product. This is to ensure both and have the terms with same ’s.
Write as
[TABLE]
where . This is because the Dirichlet coefficients of are non-negative. Now define
[TABLE]
We also define
[TABLE]
Notice that since we are working with truncated Euler-products, everything is absolutely convergent. Now, using the fact that the Fourier coefficients of are positive and that are completely multiplicative, we get the inner sum of as
[TABLE]
Thus, we have
[TABLE]
Using the generalized Merten’s Theorem 2.1, we have
[TABLE]
The second product in (3) can be bounded as follows.
[TABLE]
[TABLE]
In other words, we have
[TABLE]
Hence, we conclude
[TABLE]
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