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The Euler numbers and recursive properties of Dirichlet L-functions
Advances in Difference Equations volume 2018, Article number: 397 (2018)
Abstract
The aim of this paper is using an elementary method and the properties of the Bernoulli polynomials to establish a close relationship between the Euler numbers of the second kind \(E_{n}^{*}\) and the Dirichlet L-function \(L(s,\chi )\). At the same time, we also prove a new congruence for the Euler numbers \(E_{n}\). That is, for any prime \(p\equiv 1\bmod 8\), we have \(E_{\frac{p-3}{2}}\equiv 0\bmod p\). As an application of our result, we give a new recursive formula for one kind of Dirichlet L-functions.
1 Introduction
For any integer \(n\geq 0\) and real number \(0\leq x<1\), the Euler polynomials \(E_{n}(x)\) (see [1, 2] and [3]) and the Bernoulli polynomials \(B_{n}(x)\) (see [2, 4] and [5]) are defined by the coefficients of the power series
When \(x=0\), \(E_{n}=E_{n}(0)\) is called the nth Euler number, \(B_{n}=B_{n}(0)\) is called the nth Bernoulli number. For example, the initial values of \(E_{n}\) and \(B_{n}\) are \(E_{0}=1, E_{1}=- \frac{1}{2}, E_{2}=0, E_{3}=\frac{1}{4}, E_{4}=0, E_{5}=- \frac{1}{2}, E_{6}=0, \dots \) ; \(B_{0}=1, B_{1}=-\frac{1}{2}, B_{2}=\frac{1}{6}, B_{3}=0, B_{4}=-\frac{1}{30}, B_{5}=0, B_{6}=\frac{1}{42}, \dots \) .
The Euler numbers of the second kind \(E_{n}^{*}\) (see [2, 6, 7] and [8]) are also defined by the coefficients of the power series
where \(E_{0}^{*}=1\), \(E_{2}=-1\), \(E_{4}^{*}=5\), \(E_{6}^{*}=-61\), and \(E_{2i+1}^{*}=0\) for all integers \(i\geq 0\).
It is clear that \(E_{n}^{*}=2^{n}\cdot E_{n} ( \frac{1}{2} ) \). These polynomials and numbers arise in many combinatorial and number theory contexts. As for the elementary properties of these sequences, various authors have studied them and obtained many interesting results. For example, W. Zhang [9] obtained some combinational identities. As an application of the result in [9], he proved that for any prime p, one has the congruence
Richard K. Guy [10] (see problem B45 and [11]) proposed the following two problems:
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Is it true that for any prime \(p\equiv 1\bmod 8\), \(p\nmid E_{ \frac{p-1}{2}}^{*}\)? Is the same true for \(p\equiv 5\bmod 8\)?
G. Liu [6] solved the second problem by an elementary method. Later, W. Zhang and Z. Xu [7] solved the above two problems completely. In fact, they proved the following general conclusion: For any prime \(p\equiv 1\bmod 4\) and positive integer α, one has the congruence
where \(\phi (n)\) denotes the Euler function.
Recently, J. Zhao and Z. Chen [12] proved the following conclusion: For any positive integers n and \(k\geq 2\), one has the identity
where the sequence \(\{C(k,i)\}\) is defined as follows: For any positive integer k and integers \(0\leq i\leq k\), we define \(C(k, 0)=1\), \(C(k,k)=k!\) and
provided \(C(k,i)=0\), if \(i>k\).
As corollaries of this result, J. Zhao and Z. Chen [12] also obtained the following results: For any odd prime p, one has the congruences
T. Kim et al. (see [2, 13–17] and [18]) also obtained many interesting identities related to \(E_{n}\) and \(E_{n}^{*}\). Especially in [19], T. Kim also proved a series of important conclusions involving Euler numbers and polynomials associated with zeta functions.
In this paper, we will use elementary methods and the properties of the Bernoulli numbers to establish a close relationship between the Euler numbers of the second kind \(E_{n}^{*}\) and the Dirichlet L-function \(L(s,\chi )\). Meanwhile, we will also prove a new congruence for the Euler numbers \(E_{n}\). That is to say, we will prove the following several facts.
Theorem 1
For any positive integer n, we have the identity
where \(\chi_{4}\) denotes the non-principal character \(\bmod\ 4\), and \(L(s,\chi_{4})\) denotes the Dirichlet L-function corresponding to \(\chi_{4}\). In fact, we have
Theorem 2
For any positive integer n, we have the identity
From Theorems 1 and 2 we may immediately deduce the following:
Corollary 1
For any positive integer n, we have the recursive formula
where \(L(1,\chi_{4})=\frac{\pi }{4}\).
Corollary 2
Let p be a prime with \(p\equiv 1\bmod 8\), then we have the congruence
Corollary 3
For any positive integer n with \((n, 3)=1\), we have the congruence
Corollary 4
For any positive integer n with \((n, 5)=1\), we have the congruence
Corollary 5
For any positive integer n with \((n, 7)=1\), we have the congruence
Corollary 6
For any positive integer n with \((n, 11)=1\), we have
From Theorem 2 we can also deduce the following identities:
Some notes
Since \(E_{n}\) is not necessarily an integer, it can still be written as \(E_{n}=\frac{H_{n}}{K_{n}}\) with \((H_{n}, K_{n})=1\). So \(E_{n}\equiv 0\bmod p\) in this paper implies that \(p\mid H_{n}\) while \(p\nmid K_{n}\).
For a prime \(p\equiv 5 \bmod 8\), whether \(E_{\frac{p-3}{2}}\equiv 0 \bmod p\) is true is an interesting open problem.
2 Several simple lemmas
In this section, we will give two simple lemmas. Hereafter, we may use facts from number theory and the properties of the Bernoulli numbers, all of which can be found in [4]. Thus we will not repeat them here.
Lemma 1
For any positive integer n and real number x, we have the identity
Proof
First from the definitions of the Euler numbers and Bernoulli polynomials we have
On the other hand, from the definition of the Bernoulli polynomials and the Euler polynomials, we also have
and
Combining (2)–(4) and comparing the coefficients of the power series, we have the identity
This proves Lemma 1. □
Lemma 2
For any positive integer n, we have the identity
where \(\chi_{4}\) denotes the non-principal character \(\bmod\ 4\).
Proof
For any real number \(0< x<1\), from [4, Theorem 12.19] we have
Taking \(x=\frac{1}{4}\) in (5), we have
This proves Lemma 2. □
3 Proofs of the theorems
In this section, we will complete the proofs of our theorems. First, we prove Theorem 1. For any positive integer m, taking \(x=\frac{1}{4}\) and \(n=2m+1\) in Lemma 1, and noting that \(B_{1}=-\frac{1}{2}\) and \(B_{2i+1}=E_{2i+1} ( \frac{1}{2} ) =0\) for all integers \(i\geq 1\), we have
or
This proves Theorem 1.
Taking \(x=0\), \(n=2m\) and \(m\geq 1\) in Lemma 1, we have
Note that \(B_{2i+1}=0\) for all \(i\geq 1\), and \(B_{1}=-\frac{1}{2}\), \(E_{0}=1\) and \(E_{2i}=0\) for all \(i\geq 1\). From (7) we have
which implies
This proves Theorem 2.
Now we prove Corollary 1. For any positive integer n, note that the power series
from the definition of \(E_{2n}^{*}\) satisfies the identity
That is, for any positive integer n, we have the identity
Combining (8) and Theorem 1, we may immediately deduce the identity
This proves Corollary 1.
To prove other corollaries, taking a prime \(p=4k+1\) and \(n= \frac{p-1}{4}\) in Theorem 2, we have
From Euler’s criterion (see [4, Theorem 9.2]) we have
where \(( \frac{*}{p} ) \) denotes the Legendre’s symbol \(\bmod\ p\).
For the Bernoulli numbers \(B_{2n}\), we also have
where \(I_{n}\) is an integer and the sum is over all primes p such that \(p-1\) divides 2n.
In fact, formula (11) was discovered in 1840 by von Staudt and Clausen (independently); see [4, Exercises for Chap. 12].
Now in (11), we let \(2n=\frac{p-1}{2}\) and \(B_{2n}=\frac{U_{2n}}{V _{2n}}\), where \(U_{2n}\) and \(V_{2n}\) are two integers with \((U_{2n}, V _{2n})=1\). Since \(p-1\nmid \frac{p-1}{2}\), from (11) we know that \((V_{2n}, p)=1\) and \(( 2^{\frac{p-1}{2}}-1 ) \cdot B_{ \frac{p-1}{2}}\) is an integer.
If \(p\equiv 1\bmod 8\), then from (9), (10) and (11) we have the congruence
This proves Corollary 2.
Corollaries 3–6 can also be easily deduced from Theorem 2 and the method used when proving Corollary 2.
This completes the proofs of all our results.
If \(p\equiv 5\bmod 8\), then we have \(2^{\frac{p-1}{2}}\equiv -1 \bmod p\) and \(p\nmid ( 2^{\frac{p-1}{2}}-1 ) \). So in this case, whether one has \(E_{\frac{p-3}{2}}\equiv 0\bmod p\) remains an open problem.
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The authors wish to express their gratitude to the editors and the reviewers for their helpful comments.
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This work is supported by the N.S.F. (11771351) of P.R. China.
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Hou, Y., Shen, S. The Euler numbers and recursive properties of Dirichlet L-functions. Adv Differ Equ 2018, 397 (2018). https://doi.org/10.1186/s13662-018-1853-y
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DOI: https://doi.org/10.1186/s13662-018-1853-y