1. Introduction
The development of analytic number theory and the classical proof of the prime number theorem is closely related to the definition of the so-called second Chebyshev function [
1,
2,
3,
4]:
where
is the von Mangoldt arithmetic function:
In (
1), the apostrophe denotes that the contribution of the term
is
. Closely related with
is the first Chebyshev function, which involves only a sum over prime numbers,
p, instead of the powers of the primes:
These two functions are related by
By using the Möbius function, this equation can also be inverted, yielding the first Chebyshev function in terms of the second one:
As part of the theory of Riemann’s z. f.,
, the following estimation of
has been obtained [
5,
6]:
where
denotes the derivative of Riemann’s z. f. and
X,
T are large real parameters. The first sum on the right-hand side of (
6) extends over all the non-trivial zeros
of Riemann’s z. f., and
T can be chosen freely. Here,
is an error term bounded as follows [
6]:
Here,
denotes the integer part of
X. As
and the second sum in (
6) is
, the dominant contribution to the error of the approximation
for
is given by the sum over the non-trivial zeros. To estimate these sums, we will assume throughout this paper that a generally weaker version of Riemann’s hypothesis holds. We will call this condition the restricted Riemann’s hypothesis (RRH):
Definition 1. Restricted Riemann’s hypothesis (RRH): Let , . We say that the RRH is verified for this parameter α if for any non-trivial zero of we have . If then we recover the standard Riemann’s hypothesis.
Although it was not then known by this name, the RRH has already been considered by Ingham in his treatise on the distribution of prime numbers [
1]. Assuming RRH, for a given parameter
, we can find the bound:
Now, we denote by
the number of zeros in the critical strip with imaginary part
. This allows us to replace the sum over
by an integral:
Estimates of
were obtained by von Mangoldt in 1905 and by Backlund in 1918, following a conjecture by Riemann himself [
6]. These results imply that
for
. Combining this result with (
9), we find that
Finally, we use (
6), (
7) and (
10) with
to obtain the estimate of the second Chebyshev function:
which is equivalent to the prime number theorem proved by Hadamard and de la Vallée Poussin in 1896 [
1,
2]. This is indeed a stronger result because it assumes the validity of the still unproven RRH for a given
, while in the classical original proof only the previously proven result about the absence of zeros of
with
was taken into account.
Inspired by Equations (
1) and (
3), we propose the following generalization, to be called the first Chebyshev function of order
n:
where
, 2,
… and
p denotes the prime numbers. Similarly, we define the second Chebyshev function of order
n as a generalization of (
1), as follows:
where the apostrophe means, as before, that for
(if
X is an integer) the last term in the sum contributes only
. Here,
is the von Mangoldt function of order
, 2,
… that we will define as follows:
Note that for
we recover the standard von Mangoldt’s function. The relation of the first Chebyshev function of order
n and the second Chebyshev function of the same order can be found in terms of a Möbius inversion formula:
which allows us to estimate the sums of the powers of the logarithm of the prime numbers once the second Chebyshev function of order
n is also estimated.
The objective of this paper is to extend the classical techniques of analytical number theory, which allow us to estimate the Chebyshev functions, to the case of the generalized Chebyshev functions as defined in Equations (
12) and (
13). The paper is organized as follows: In
Section 2, we discuss the analytical calculation of the Chebyshev functions of order
and make some indications for the cases
, 4,
… Some theorems about Riemann’s z. f., which are required in the proof of
Section 2, are enumerated in the
Appendix A. The results in
Section 2 suggest a heuristic approach to calculating general divergent sums of arithmetic functions over the primes, which is presented in
Section 3. We also apply this conjecture to evaluating the sum of the logarithm of the logarithm of the prime numbers. The paper ends with some conclusions in
Section 4.
2. The Sum of the Squares of the Logarithm of the Prime Numbers
In this section, we evaluate an approximation to the first and second Chebyshev functions of order
that, according to (
15), are closely related. Our objective, therefore, is to prove the following theorem:
Theorem 1. Under the assumption of the restricted Riemann hypothesis (RRH), the following result for the sum of the squares of the logarithm of prime numbers, , holds:where is a real number corresponding to the strip , where all non-trivial zeros of are located. We should start by defining the following piecewise function:
This function can also be represented as an integral in the complex plane:
where
is a real constant and
. This integral can also be defined as the limit, as
, of the following integral over a segment in the complex plane:
i.e.,
. The approach of
to
is algebraic in
T as
T becomes large. In [
6], it is proven that
Now, we have the necessary tool to find a connection among the Chebyshev functions of order
and Riemann’s z. f. From the definition in (
13) and (
17), we have
which, obviously, is the limit, as
, of
From Equations (
19) to (
22), we can find an integral representation of
:
Absolute convergence for
allows us to swap the sum and the integral in (
23), yielding
Here, the sum in parentheses can be evaluated in terms of Riemann’s z. f. To see the connection, we start with Euler’s product:
where, as usual,
p denotes all the prime numbers. From (
25), we can find the second-order logarithmic derivative of
, as follows:
This allows us to rewrite (
24) in the following form:
This equation explicitly states the connection between the second Chebyshev function of order and Riemann’s z. f.
Our next objective is to find an asymptotic expression for
, estimating the error term. The first step, then, is to calculate the distance between
and
. From Equations (
20)–(
22), we obtain
where the first term on the right-hand side of the inequality only appears when
X is a prime power. As
X is expected to be large, we assume
to take
c as follows:
The second sum on the right-hand side of (
28) is better estimated by separating the summation into four regions. To this end, we define
And the sum in (
28) can be written as
To proceed, we need Lemma A1 in the
Appendix A. Firstly, we analyze the first and last sums on the right-hand side of (
31). If we have
or
then the following inequality holds:
Then, from Equations (
29) and (
30) and Lemma A1, we have
We now consider the sum
. We denote by
the largest prime power less than
X. In the following, we assume that
lies in the interval
. If there is no prime power in this interval then
and the whole sum
is bounded by the term in (
33). But the contribution of the prime powers in this interval may be large, and we need to take that into account. For the term
, we have
Therefore, the contribution of this term is bounded by
where we have taken into account that for the prime power
and, according to the definition in (
14), we have
For the rest of the prime powers in the interval
, we can say that
, with
m being a subset of
. In this case, the following inequality holds:
Consequently, this contribution to
would be bounded by
We now consider the sum of the reciprocals of the integers:
Here,
denotes the Euler–Mascheroni constant. From Equations (
35), (
38) and (
39), we then have
A similar argument can be applied to the remaining partial summation, i.e.,
. This would lead to the same result as given in (
40). Therefore, from Equations (
33) and (
40), we obtain
Our next objective is to evaluate
and, by taking the limit
, also
. We begin with an identity for the logarithmic derivative of Riemann’s z. f.:
Here, the sum over
extends over all non-trivial zeros of Riemann’s z. f. Absolute convergence allows us to differentiate term-by-term the expression in (
42):
The sums in (
43) are convergent for any value of
s. To evaluate the integral in (
27), we use Cauchy’s residue theorem on the path plotted in
Figure 1.
The corresponding integral along this path can be written as follows:
We note that the poles inside the path
are
,
and
(
being a zero of Riemann’s z. f. with imaginary part
) and
, with
n and integer in the interval
. Both
T and
U are chosen in such a way that there are no zeros along the path
. By Cauchy’s residue theorem, we have
The last summation in (
45), as
, can be written in terms of the polylogarithmic functions:
Here,
is the logarithmic function and
is the dilogarithmic function. Although it is not necessary for our calculation, we can also give an explicit expression for the constant term in (
45), as follows [
5]:
where
is the Euler–Mascheroni constant and
is the Stieltjes constant of order one.
From Cauchy’s integral theorem, we can now write
To estimate the order of magnitude of these integrals, we should study the integrands in several domains. Firstly, we consider (for
and
) that
and
.
T also being different from the imaginary part of a zero of Riemann’s z. f.,
. From (
42), we can also write
where
represents the non-trivial zeros of Riemann’s z. f. From Stirling’s formula [
5], we have
and, consequently,
as
. And now, taking the logarithmic derivative,
and also
for
and
, this yields
From Equations (
50) and (
55), we have
To proceed, we now need Theorem A1 about the sums of the reciprocal of the squares of the distances of the imaginary part of the non-trivial zeros of Riemann’s z. f. from
T for
(see
Appendix A). This way, we arrive at
We also need the Theorem A2 discussed by Chen [
6]: for all sufficiently large positive real numbers
T, the number of zeros of the function
in the critical strip with
is
. As
T is arbitrary, and according also to this theorem, we can choose it in such a way that
for any nontrivial zero of
with imaginary part
. Consequently, in the aforementioned region of the complex plane, we have
and, as the number of zeros of
such that
is
, we finally arrive at the following estimate:
valid for
,
. We also need another estimate of the second-order logarithmic derivative of
for
and
for
. We start with the unsymmetrical form of the functional equation [
5,
6,
7]:
or, equivalently,
Stirling’s formula for
yields
On the other hand, the last two terms in (
61) are bounded for
, and we have
We are now ready to evaluate the integrals on the right-hand side of (
49). The first integral is separated for convenience, as follows:
From Equations (
29) and (
59), we have
Similarly, by using (
63), we can write
Note that the results in Equations (
65) and (
66) are also valid if we replace
T by
. This means that the third integral on the right-hand side of (
49) has the same value as the first one. Finally, we evaluate a bound for the second integral:
under the conditions
,
for all
and also
(which is true for
). Thus, the bound in (
63) can be applied again, to obtain
Now, from (
49) and the inequalities in Equations (
65)–(
68), we arrive at
If we take the limit
and use Equations (
45) and (
46) then the following expression for
is deduced:
where
C is the constant given in (
48). The Chebyshev function of order 1 is now calculated from Equations (
41) and (
70), yielding
with the following error term:
To obtain our estimation of , we need first to evaluate a bound on the sums over the nontrivial zeros of Riemann’s z. f. In this calculation, we assume the validity of the restricted Riemann’s hypothesis in Definition 1 for a given real number, .
Under these conditions, we have
where we have applied the principle that the sum of the reciprocals of all the nontrivial zeros of Riemann’s z. f. is a constant [
5]. Similarly,
Here, we have used (
10), about the sums of the reciprocals of the imaginary part of the nontrivial zeros of Riemann’s z. f. From Equations (
71)–(
74), we finally deduce that
with the error term
given by
In (
76), we have taken into account that the large prime power lesser or equal to
X,
, is greater than 1, and we have included only the dominant contributions. Note, again, that
X and
T are large but independent. The error term in (
76) can then be optimized for fixed
X by finding its minimum as a function of
T. Approximately, this is equivalent to taking
for
. With this choice of
T, we finally obtain our estimation of the Chebyshev function of order 1 under the assumption of the validity of the restricted Riemann’s hypothesis:
For
, the behavior of
given in this equation is equivalent to Riemann’s hypothesis. Now, by using the Möbius inversion formula in (
15), we deduce that the first Chebyshev function of order 1 behaves asymptotically in the same way as
, i.e., the sums of the squares of the logarithm of the primes lesser or equal to
X are also given by the asymptotic expression in (
77). This proves Theorem 1.