1. Introduction
It has been almost 164 years since the Riemann Hypothesis (RH) was proposed in 1859 [
1]. Many efforts and achievements have been made towards proving this celebrated hypothesis, but it is still an open problem [
2,
3].
The Riemann zeta function is the function of the complex variable
s, defined in the half-plane
by the absolutely convergent series [
2]
The connection between the Riemann zeta function and prime numbers can be established through the well-known Euler product, i.e.
where
p runs over the prime numbers.
Riemann showed how to extend zeta function to the whole complex plane
by analytic continuation
where
being the Jaccobi theta function,
being the Gamma function in the following Weierstrass expression (Meanwhile, there are also Gauss expression, Euler expression, and integral expression of the Gamma function.)
where
is the Euler-Mascheroni constant.
As shown by Riemann,
extends to
as a meromorphic function with only a simple pole at
, with residue 1, and satisfies the following functional equation
The Riemann zeta function
has zeros at the negative even integers:
,
,
,
, ⋯ and one refers to them as the
trivial zeros. The other zeros of
are the complex numbers, i.e.,
non-trivial zeros [
2].
In 1896, Hadamard [
4] and Poussin [
5] independently proved that no zeros could lie on the line
. Together with the functional equation and the fact that there are no zeros with real part greater than 1, this showed that all non-trivial zeros must lie in the interior of the
critical strip. This was a key step in their first proofs of the famous
Prime Number Theorem.
Later on, Hardy (1914) [
6], Hardy and Littlewood (1921) [
7] showed that there are infinitely many zeros on the
critical line , which was an astonishing result at that time.
As a summary, we have the following results on the properties of the non-trivial zeros of
[
4,
5,
6,
7,
8,
9].
Lemma 1. Non-trivial zeroes of , noted as , have the following properties
1) The number of non-trivial zeroes is infinity;
2) ;
3) ;
4) are all non-trivial zeroes.
As further study, a completed zeta function
is defined as
It is well-known that is an entire function of order 1. This implies is analytic, and can be expressed as infinite polynomial, in the whole complex plane .
In addition, replacing
s with
in Equation (
6), and combining Equation (
5), we have the following functional equation
Considering the definition of
, and recalling Equation (
4), the trivial zeros of
are canceled by the poles of
. The zero of
and the pole of
cancel; the zero
and the pole of
cancel [
9,
10]. Thus, all the zeros of
are exactly the nontrivial zeros of
. Then we have the following Lemma 2.
Lemma 2. The zeros of coincide with the non-trivial zeros of .
According to Lemma 2, the following two statements for the RH are equivalent.
Statement 1 of the RH: All the non-trivial zeros of have real part equal to .
Statement 2 of the RH: All the zeros of have real part equal to .
To prove the RH, a natural thinking is to estimate the numbers of non-trivial zeros of
inside or outside some areas according to Argument Principle. Along this train of thought, there are many research works. Let
denote the number of zeros of
inside the rectangle:
, and let
denote the number of zeros of
on the line
. Selberg proved that there exist positive constants
c and
, such that
[
11], later on, Levinson proved that
[
12], Lou and Yao proved that
[
13], Conrey proved that
[
14], Bui, Conrey and Young proved that
[
15], Feng proved that
[
16], Wu proved that
[
17].
On the other hand, many zeros have been calculated by hand or by computer programs. Among others, Riemann found the first three non-trivial zeros [
18]. Gram found the first 15 zeros based on Euler-Maclaurin summation [
19]. Titchmarsh calculated the 138
th to 195
th zeros using the Riemann-Siegel formula [
20,
21]. Here are the first three (pairs of) zeros:
.
The idea of this paper is originated from Euler’s work on proving the following famous equality
This interesting result is deduced by comparing the like terms of two types of infinite expressions, i.e., infinite polynomial and infinite product, as shown in the following
Then it is conjectured that
should be factored into
or something like that, which was verified by paring
and
in the Hadamard product of
to obtain
The Hadamard product of
as shown in Equation (
10) was first proposed by Riemann, however, it was Hadamard [
22] who showed the validity of this infinite product expansion.
where
,
runs over all the non-trivial zeros of the Riemann zeta function
, or in another word,
runs over all the zeros of the completed zeta function
.
To ensure the absolute convergence of the infinite product expansion,
and
are paired. Later in
Section 4, we will show that
and
can also be paired to ensure the absolute convergence of the infinite product expansion.
2. Lemmas
In this section, we first explain the concepts of multiple zeros of with their real multiplicities. And then we give three lemmas to support the proof of the RH, in which Lemma 3 is the key lemma.
Multiple zeros of : As shown in
Figure 1, the multiple zeros of
are defined in terms of the quadruplet, i.e.,
.
It should be noticed that the multiple zeros with their real multiplicities of
are objective existence, but the expression of the corresponding factors of
are optional to some extent. For example, the multiple zeros as shown in
Figure 1 have two different expressions as factors of
and
, respectively, i.e.,
, or
.
To exclude the latter expression, we stipulate that the multiple zeros related factors of take the form of , where is the real multiplicity of .
Lemma 3.
Given two infinite products
and
where s is a complex variable, and are the complex conjugate zeros of , and are real numbers, are the real multiplicities of , i are natural numbers from 1 to infinity, are in order of increasing , i.e., .
Then we have
where is the equivalent sign.
Proof. First of all, we have the following fact:
where
is a natural number,
and
are real numbers.
Next, the proof is based on Transfinite Induction. Let
be:
According to Equation (
14),
is an obvious fact as the
Base Case, i.e.,
To be more convincing, let’s further check
, i.e.,
which is also an obvious fact according to Lemma 5.
As the Successor Case, we need to prove .
Actually, we have
Thus the
Successor Case is true, i.e.,
.
Next, we prove that holds by considering well-ordered ordinal set A indexing the family of statements , with the ordering that for all natural numbers n, is the first limit ordinal.
It is well-known that .
To prove that holds, it suffices to prove the Limit Case, i.e., .
Actually, we have
Thus the
Limit Case is true, i.e.,
.
Hence we conclude by Transfinite Induction that
holds, i.e.,
i.e.,
That completes the proof of Lemma 3. □
Lemma 4.
Given
where s is a complex variable, and are real numbers, .
Proof. Expanding both sides of Equation (
22), and comparing the coefficients of like terms, we obtain (details are omitted to save space)
The inverse inference of Equation (
24) is also an obvious fact. i.e.,
Then we have
Further, according to Equation (
14), i.e.,
and the following similar facts
we have
That completes the proof of Lemma 4. □
Remark 1. From the viewpoint of the products of (polynomial) factors, Lemma 4 reveals a fact that there are only two possible cases to make Eq. (22) hold, i.e., Case 1: , ; and Case 2: , , respectively. This result is quite different from the similar equation of products of factors , which has infinite cases of solutions for real numbers .
Remark 2.
Given
where s is a complex variable, and are real numbers, are natural numbers, denoting the real multiplicities of and , respectively, .
Proof. Based on Lemma 4, and considering additional possibilities that
,
(where "∣" is the divisible sign), or vice versa, we have
That completes the proof of Lemma 5. □
3. A Proof of the RH
This section is planned to present a proof of the Riemann Hypothesis. We first prove that Statement 2 of the RH is true, and then by Lemma 2, Statement 1 of the RH is also true.
Proof of the RH. The details are delivered in three steps as follows.
Step 1: It is well-known that all the zeros of
always come in complex conjugate pairs. Then by pairing
and
in the Hadamard product as shown in Equation (
10), we have
where
.
The absolute convergence of the infinite product in Equation (
30) in the form
depends on the convergence of infinite series
, or equivalently,
, which is an obvious fact according to Theorem 2 in
Section 2, Chapter IV of Ref. [
23], as shown in the following.
Theorem 1. ([
23])
.The function is an entire function of order one that has infinitely many zeros such that . The series diverges, but the series converges for any . The zeros of are the nontrivial zeros of .
Remark 3. In the Theorem 2 of Ref. [23], is identical to in this paper, both and mean the real part of any complex number.
Further, considering the absolute convergence of
we have the following new expression of
by putting all the
related multiple factors (zeros) together in the above Equation (
32)
where
are the real multiplicities of
,
i are natural numbers from 1 to infinity.
Step 2: Replacing
s with
in Equation (
33), we obtain the infinite product expression of
, i.e.,
Remark 4. According to the new expressions of and , i.e., Equation (33) and Equation (34), we may conclude that all and related multiple zeros, i.e., are included in the group of factors, and , respectively, or in another word, before or after the group of factors of and , there are no and related multiple zeros.
Actually, with such arrangement of and related multiple factors of and , we "assigned" a reason for excluding, in the proof of Lemma 5 and Lemma 3, the "abnormal" situation, i.e., the successor factor and its predecessor factor represent the same quadruplet of zeros.
Step 3: According to the functional equation
, and considering Equation (
33) and Equation (
34), we have
which is equivalent to
And that
can be certainly arranged in order of increasing
, i.e.,
.
Then, according to Lemma 3, Equation (
36) is equivalent to
Thus, we conclude that all the zeros of the completed zeta function
have real part equal to
, i.e., Statement 2 of the RH is true. According to Lemma 2, Statement 1 of the RH is also true, i.e., all the non-trivial zeros of the Riemann zeta function
have real part equal to
.
That completes the proof of the RH. □
Remark 5. By Lemma 1, there are 2 pairs of complex zeros of simultaneously, i.e., are all the non-trivial zeroes of . With the proof of the RH, i.e., , these 2 pairs of zeros are actually only one pair, because . Thus Lemma 1 could be modified more precisely as follows.
Lemma 5. Non-trivial zeroes of , noted as , have the following properties
1) The number of non-trivial zeroes is infinity;
2) ;
3) ;
4) are all non-trivial zeroes.