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 (this does not refer to the number π).
in the sense that
This statement is the prime number theorem. An equivalent statement is
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.
More precise estimates of are now known; for example
where the O is big O notation. For most values of we are interested in (i.e., when is not unreasonably large) is greater than , but infinitely often the opposite is true. For a discussion of this, see Skewes' number.
Table of π(x), x / ln x, and li(x)
x π(x) π(x) − x / ln x li(x) − π(x) x / π(x) 10 4 −0.3 2.2 2.500 102 25 3.3 5.1 4.000 103 168 23 10 5.952 104 1,229 143 17 8.137 105 9,592 906 38 10.425 106 78,498 6,116 130 12.740 107 664,579 44,158 339 15.047 108 5,761,455 332,774 754 17.357 109 50,847,534 2,592,592 1,701 19.667 1010 455,052,511 20,758,029 3,104 21.975 1011 4,118,054,813 169,923,159 11,588 24.283 1012 37,607,912,018 1,416,705,193 38,263 26.590 1013 346,065,536,839 11,992,858,452 108,971 28.896 1014 3,204,941,750,802 102,838,308,636 314,890 31.202 1015 29,844,570,422,669 891,604,962,452 1,052,619 33.507 1016 279,238,341,033,925 7,804,289,844,393 3,214,632 35.812 1017 2,623,557,157,654,233 68,883,734,693,281 7,956,589 38.116 1018 24,739,954,287,740,860 612,483,070,893,536 21,949,555 40.420 1019 234,057,667,276,344,607 5,481,624,169,369,960 99,877,775 42.725 1020 2,220,819,602,560,918,840 49,347,193,044,659,701 222,744,644 45.028 1021 21,127,269,486,018,731,928 446,579,871,578,168,707 597,394,254 47.332 1022 201,467,286,689,315,906,290 4,060,704,006,019,620,994 1,932,355,208 49.636 1023 1,925,320,391,606,803,968,923 37,083,513,766,578,631,309 7,250,186,216 51.939 1024 18,435,599,767,349,200,867,866 339,996,354,713,708,049,069 17,146,907,278 54.243 1025 176,846,309,399,143,769,411,680 3,128,516,637,843,038,351,228 55,160,980,939 56.546
In the On-Line Encyclopedia of Integer Sequences, the π(x) column is sequence A006880, π(x) - x / ln x is sequence A057835, and li(x) − π(x) is sequence A057752. The value for π(1024) was originally computed by J. Buethe, J. Franke, A. Jost, and T. Kleinjung assuming the Riemann hypothesis. It has since been verified unconditionally in a computation by D. J. Platt.
Algorithms for evaluating π(x)
A simple way to find , if is not too large, is to use the sieve of Eratosthenes to produce the primes less than or equal to and then to count them.
A more elaborate way of finding is due to Legendre: given , if are distinct prime numbers, then the number of integers less than or equal to which are divisible by no is
(where denotes the floor function). This number is therefore equal to
when the numbers are the prime numbers less than or equal to the square root of .
In a series of articles published between 1870 and 1885, Ernst Meissel described (and used) a practical combinatorial way of evaluating . Let , be the first primes and denote by the number of natural numbers not greater than which are divisible by no . Then
Given a natural number , if and if , then
Using this approach, Meissel computed , for equal to 5×105, 106, 107, and 108.
In 1959, Derrick Henry Lehmer extended and simplified Meissel's method. Define, for real and for natural numbers , and , as the number of numbers not greater than m with exactly k prime factors, all greater than . Furthermore, set . Then
where the sum actually has only finitely many nonzero terms. Let denote an integer such that , and set . Then and when ≥ 3. Therefore
The computation of can be obtained this way:
On the other hand, the computation of can be done using the following rules:
Using his method and an IBM 701, Lehmer was able to compute .
Further improvements to this method were made by Lagarias, Miller, Odlyzko, Deléglise and Rivat.
and setting , Laplace-transforming both sides and applying a geometric sum on got the expression
Other prime-counting functions
Other prime-counting functions are also used because they are more convenient to work with. One is Riemann's prime-counting function, usually denoted as or . This has jumps of 1/n for prime powers pn, with it taking a value half-way between the two sides at discontinuities. That added detail is because then it may be defined by an inverse Mellin transform. Formally, we may define by
where p is a prime.
We may also write
where Λ(n) is the von Mangoldt function and
Möbius inversion formula then gives
The Chebyshev function weights primes or prime powers pn by ln(p):
Riemann's prime-counting function has the ordinary generating function:
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:
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 we have a more complicated formula
Again, the formula is valid for x > 1, while ρ are the nontrivial zeros of the zeta function ordered according to their absolute value, and, again, the latter integral, taken with minus sign, is just the same sum, but over the trivial zeros. The first term li(x) is the usual logarithmic integral function; the expression li(xρ) in the second term should be considered as Ei(ρ ln x), where Ei is the analytic continuation of the exponential integral function from positive reals to the complex plane with branch cut along the negative reals.
valid for x > 1, where
The sum over non-trivial zeta zeros in the formula for describes the fluctuations of , while the remaining terms give the "smooth" part of prime-counting function, so one can use
as the best estimator of for x > 1.
The amplitude of the "noisy" part is heuristically about , so the fluctuations of the distribution of primes may be clearly represented with the Δ-function:
An extensive table of the values of Δ(x) is available.
Here are some useful inequalities for π(x).
- for x ≥ 17.
The left inequality holds for x ≥ 17 and the right inequality holds for x > 1.
Here are some inequalities for the nth prime, pn.
- for n ≥ 6.
The left inequality holds for n ≥ 1 and the right inequality holds for n ≥ 6.
An approximation for the nth prime number is
The Riemann hypothesis
The Riemann hypothesis is equivalent to a much tighter bound on the error in the estimate for , and hence to a more regular distribution of prime numbers,
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