Prime-counting function

In mathematics, the prime-counting function is the function counting the number of prime numbers less than or equal to some real number ''x''. It is denoted by (''x'') (unrelated to the number ).

History

Of great interest in number theory is the growth rate of the prime-counting function. It was conjectured in the end of the 18th century by Gauss and by Legendre to be approximately

:$\frac x$

where log is the natural logarithm, in the sense that

:$\lim_ \frac=1.$

This statement is the prime number theorem. An equivalent statement is

:$\lim_\pi(x) / \operatorname(x)=1$

where li is the logarithmic integral function. The prime number theorem was first proved in 1896 by Jacques Hadamard and by Charles de la Vallée Poussin independently, using properties of the Riemann zeta function introduced by Riemann in 1859. Proofs of the prime number theorem not using the zeta function or complex analysis were found around 1948 by Atle Selberg and by Paul Erdős (for the most part independently).

In 1899, de la Vallée Poussin proved that
(see also Theorem 23 of)

:$\pi(x) = \operatorname (x) + O \left(x e^\right) \quad\text x \to \infty$

for some positive constant . Here, is the big notation.

More precise estimates of $\pi(x)\!$ are now known. For example, in 2002, Kevin Ford proved that

:$\pi(x) = \operatorname (x) + O \left(x \exp \left( -0.2098(\log x)^\frac35 (\log \log x)^ \right) \right).$

Mossinghoff and Trudgian proved an explicit upper bound for the difference between $\pi(x)$ and $\operatorname(x)$:
:$\big, \pi(x) - \operatorname(x) \big, \le 0.2593 \frac \exp \left( -\sqrt \right)$
for $x \ge 229$.

For most values of $x$ we are interested in (i.e., when $x$ is not unreasonably large) $\operatorname(x)$ is greater than $\pi(x)$. However, $\pi(x) - \operatorname(x)$ is known to change sign infinitely many times. For a discussion of this, see Skewes' number.

Exact form

For $x>1$ let $\pi_0 (x)=\pi(x)-1/2$ when $x$ is a prime number, and $\pi_0 (x)=\pi(x)$ otherwise. Of profound importance, Bernhard Riemann proved that $\pi_0(x)$ is equal to

:$\pi_0(x) = \operatorname(x) - \sum_\operatorname(x^\rho)$

where

:$\operatorname(x) = \sum_^ \frac \operatorname(x^),$

is the Möbius function, is the logarithmic integral function, ''ρ'' indexes every zero of the Riemann zeta function, and is not evaluated with a branch cut but instead considered as where is the exponential integral. If the trivial zeros are collected and the sum is taken ''only'' over the non-trivial zeros ''ρ'' of the Riemann zeta function, then $\pi_0(x)$ may be approximated by

:$\pi_0(x) \approx \operatorname(x) - \sum_\operatorname(x^\rho) - \frac + \frac \arctan .$

The Riemann hypothesis suggests that every such non-trivial zero lies along .

Table of (''x''), ''x'' / log ''x'', and li(''x'')

The table shows how the three functions (''x''), ''x'' / log ''x'' and li(''x'') compare at powers of 10. See also, and

:

In the On-Line Encyclopedia of Integer Sequences, the (''x'') column is sequence , is sequence , and is sequence .

The value for (10²⁴) was originally computed by J. Buethe, J. Franke, A. Jost, and T. Kleinjung assuming the Riemann hypothesis.
It was later verified unconditionally in a computation by D. J. Platt.
The value for (10²⁵) is due to J. Buethe, J. Franke, A. Jost, and T. Kleinjung.
The value for (10²⁶) was computed by D. B. Staple. All other prior entries in this table were also verified as part of that work.

The value for 10²⁷ was announced in 2015 by David Baugh and Kim Walisch.

The value for 10²⁸ was announced in 2020 by David Baugh and Kim Walisch.

The value for 10²⁹ was announced in 2022 by David Baugh and Kim Walisch.

Algorithms for evaluating (''x'')

A simple way to find $\pi(x)$, if $x$ is not too large, is to use the sieve of Eratosthenes to produce the primes less than or equal to $x$ and then to count them.

A more elaborate way of finding $\pi(x)$ is due to Legendre (using the inclusion–exclusion principle): given $x$, if $p_1,p_2,\ldots,p_n$ are distinct prime numbers, then the number of integers less than or equal to $x$ which are divisible by no $p_i$ is

:$\lfloor x\rfloor - \sum_\left\lfloor\frac\right\rfloor + \sum_ \left\lfloor\frac\right\rfloor - \sum_\left\lfloor\frac\right\rfloor + \cdots$

(where $\lfloor\rfloor$ denotes the floor function). This number is therefore equal to

:$\pi(x)-\pi\left(\sqrt\right)+1$

when the numbers $p_1, p_2,\ldots,p_n$ are the prime numbers less than or equal to the square root of $x$.

The Meissel–Lehmer algorithm

In a series of articles published between 1870 and 1885, Ernst Meissel described (and used) a practical combinatorial way of evaluating $\pi(x)$. Let $p_1, p_2, \ldots, p_n$ be the first $n$ primes and denote by $\Phi(m,n)$ the number of natural numbers not greater than $m$ which are divisible by no $p_i$ where $i\leq n$. Then

: $\Phi(m,n)=\Phi(m,n-1)-\Phi\left(\frac m ,n-1\right).$

Given a natural number $m$, if n=\pi\left(\sqrt right) and if $\mu = \pi\left(\sqrt\right)-n$, then

:$\pi(m)=\Phi(m,n)+n(\mu+1)+\frac 2 - 1 - \sum_^\mu\pi\left(\frac m \right).$

Using this approach, Meissel computed $\pi(x)$, for $x$ equal to 5, 10⁶, 10⁷, and 10⁸.

In 1959, Derrick Henry Lehmer extended and simplified Meissel's method. Define, for real $m$ and for natural numbers $n$ and $k$, $P_k(m,n)$ as the number of numbers not greater than ''m'' with exactly ''k'' prime factors, all greater than $p_n$. Furthermore, set $P_0(m,n)=1$. Then

:$\Phi(m,n) = \sum_^ P_k(m,n)$

where the sum actually has only finitely many nonzero terms. Let $y$ denote an integer such that \sqrt le y\le\sqrt, and set $n=\pi(y)$. Then $P_1(m,n)=\pi(m)-n$ and $P_k(m,n)=0$ when $k \geq 3$. Therefore,

:$\pi(m)=\Phi(m,n)+n-1-P_2(m,n)$

The computation of $P_2(m,n)$ can be obtained this way:

:$P_2(m,n) = \sum_\left(\pi\left(\frac m p \right)-\pi(p)+1\right),$

where the sum is over prime numbers.

On the other hand, the computation of $\Phi(m,n)$ can be done using the following rules:

#$\Phi(m,0)=\lfloor m\rfloor$
#$\Phi(m,b) = \Phi(m,b-1) - \Phi\left(\frac m,b-1\right)$

Using his method and an IBM 701, Lehmer was able to compute the correct value of $\pi\left(10^\right)$ and missed the correct value of $\pi\left(10^\right)$ by 1.

Further improvements to this method were made by Lagarias, Miller, Odlyzko, Deléglise and Rivat.

Other prime-counting functions

Other prime-counting functions are also used because they are more convenient to work with. One is Riemann's prime-power counting function, usually denoted as $\Pi_0(x)$ or $J_0(x)$. This has jumps of 1/''n'' for prime powers ''p''^''n'', with it taking a value halfway between the two sides at discontinuities. That added detail is used because then the function may be defined by an inverse Mellin transform. Formally, we may define $\Pi_0(x)$ by

:$\Pi_0(x) = \frac 1 2 \left( \sum_ \frac 1 n \ + \sum_ \frac 1 n \right)$

where ''p'' is a prime.

We may also write

:$\Pi_0(x) = \sum_^x \frac - \frac = \sum_^\infty \frac 1 n \pi_0\bigl(x^\bigr)$

where $\Lambda(n)$ is the von Mangoldt function and

:$\pi_0(x) = \lim_ \frac 2.$

The Möbius inversion formula then gives

:$\pi_0(x) = \sum_^\infty \fracn \Pi_0\bigl(x^\bigr)$

Knowing the relationship between the logarithm of the Riemann zeta function and the von Mangoldt function $\Lambda$, and using the Perron formula we have

:$\log \zeta(s) = s \int_0^\infty \Pi_0(x) x^ \,dx$

The Chebyshev function weights primes or prime powers ''p''^''n'' by log(''p''):

:$\theta(x) = \sum_ \log p$
:$\psi(x) = \sum_ \log p = \sum_^\infty \theta\bigl(x^\bigr) = \sum_\Lambda(n).$

Formulas for prime-counting functions

Formulas for prime-counting functions come in two kinds: arithmetic formulas and analytic formulas. Analytic formulas for prime-counting were the first used to prove the prime number theorem. They stem from the work of Riemann and von Mangoldt, and are generally known as explicit formulas.

We have the following expression for ''ψ'':

:$\psi_0(x) = x - \sum_\rho \frac - \log 2\pi - \frac \log\left(1-x^\right),$

where

: $\psi_0(x) = \lim_ \frac.$

Here ''ρ'' are the zeros of the Riemann zeta function in the critical strip, where the real part of ''ρ'' is between zero and one. The formula is valid for values of ''x'' greater than one, which is the region of interest. The sum over the roots is conditionally convergent, and should be taken in order of increasing absolute value of the imaginary part. Note that the same sum over the trivial roots gives the last subtrahend in the formula.

For $\Pi_0(x)$ we have a more complicated formula

:$\Pi_0(x) = \operatorname(x) - \sum_ \operatorname(x^\rho) - \log 2 + \int_x^\infty \frac.$

Again, the formula is valid for ''x'' > 1, while ''ρ'' are the nontrivial zeros of the zeta function ordered according to their absolute value. The integral is equal to the series over the trivial zeros:

:$\int_x^\infty \frac=\int_x^\infty \frac \left(\sum_t^\right)\,\mathrm dt=\sum_\int_x^\infty \frac \,\mathrm dt \,\,\overset-\sum_ \operatorname(x^)$

The first term li(''x'') is the usual logarithmic integral function; the exp