Prime-counting function

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 ).
Growth rate
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
where log is the natural logarithm, 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. 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).
More precise estimates
In 1899, de la Vallée Poussin proved that
for some positive constant . Here, is the big notation.
More precise estimates of are now known. For example, in 2002, Kevin Ford proved that
Mossinghoff and Trudgian proved an explicit upper bound for the difference between and :
for .
For values of that are not unreasonably large, is greater than . However, is known to change sign infinitely many times. For a discussion of this, see Skewes' number.
Exact form
For let when is a prime number, and otherwise. Bernhard Riemann, in his work On the Number of Primes Less Than a Given Magnitude, proved that is equal to
where
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 may be approximated by
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
{| class="wikitable" style="text-align: right"
! x
! (x)
!
!
!
!
|-
| 10
| 4
| 0
| 2
| 2.500
| -8.57%
|-
| 102
| 25
| 3
| 5
| 4.000
|13.14%
|-
| 103
| 168
| 23
| 10
| 5.952
|13.83%
|-
| 104
| 1,229
| 143
| 17
| 8.137
|11.66%
|-
| 105
| 9,592
| 906
| 38
| 10.425
|9.45%
|-
| 106
| 78,498
| 6,116
| 130
| 12.739
|7.79%
|-
| 107
| 664,579
| 44,158
| 339
| 15.047
|6.64%
|-
| 108
| 5,761,455
| 332,774
| 754
| 17.357
|5.78%
|-
| 109
| 50,847,534
| 2,592,592
| 1,701
| 19.667
|5.10%
|-
| 1010
| 455,052,511
| 20,758,029
| 3,104
| 21.975
|4.56%
|-
| 1011
| 4,118,054,813
| 169,923,159
| 11,588
| 24.283
|4.13%
|-
| 1012
| 37,607,912,018
| 1,416,705,193
| 38,263
| 26.590
|3.77%
|-
| 1013
| 346,065,536,839
| 11,992,858,452
| 108,971
| 28.896
|3.47%
|-
| 1014
| 3,204,941,750,802
| 102,838,308,636
| 314,890
| 31.202
|3.21%
|-
| 1015
| 29,844,570,422,669
| 891,604,962,452
| 1,052,619
| 33.507
|2.99%
|-
| 1016
| 279,238,341,033,925
| 7,804,289,844,393
| 3,214,632
| 35.812
|2.79%
|-
| 1017
| 2,623,557,157,654,233
| 68,883,734,693,928
| 7,956,589
| 38.116
|2.63%
|-
| 1018
| 24,739,954,287,740,860
| 612,483,070,893,536
| 21,949,555
| 40.420
|2.48%
|-
| 1019
| 234,057,667,276,344,607
| 5,481,624,169,369,961
| 99,877,775
| 42.725
|2.34%
|-
| 1020
| 2,220,819,602,560,918,840
| 49,347,193,044,659,702
| 222,744,644
| 45.028
|2.22%
|-
| 1021
| 21,127,269,486,018,731,928
| 446,579,871,578,168,707
| 597,394,254
| 47.332
|2.11%
|-
| 1022
| 201,467,286,689,315,906,290
| 4,060,704,006,019,620,994
| 1,932,355,208
| 49.636
|2.02%
|-
| 1023
| 1,925,320,391,606,803,968,923
| 37,083,513,766,578,631,309
| 7,250,186,216
| 51.939
|1.93%
|-
| 1024
| 18,435,599,767,349,200,867,866
| 339,996,354,713,708,049,069
| 17,146,907,278
| 54.243
|1.84%
|-
| 1025
| 176,846,309,399,143,769,411,680
| 3,128,516,637,843,038,351,228
| 55,160,980,939
| 56.546
|1.77%
|-
| 1026
| 1,699,246,750,872,437,141,327,603
| 28,883,358,936,853,188,823,261
| 155,891,678,121
| 58.850
|1.70%
|-
|1027
|16,352,460,426,841,680,446,427,399
|267,479,615,610,131,274,163,365
|508,666,658,006
|61.153
|1.64%
|-
|1028
|157,589,269,275,973,410,412,739,598
|2,484,097,167,669,186,251,622,127
|1,427,745,660,374
|63.456
|1.58%
|-
|1029
|1,520,698,109,714,272,166,094,258,063
|23,130,930,737,541,725,917,951,446
|4,551,193,622,464
|65.759
|1.52%
|}
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 , 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 (using the inclusion–exclusion principle): 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 .
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 Let be the first primes and denote by the number of natural numbers not greater than which are divisible by none of the for any Then
Given a natural number if and if then
Using this approach, Meissel computed for equal to 5× , and .
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 with exactly 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 Therefore,
The computation of can be obtained this way:
where the sum is over prime numbers.
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 the correct value of and missed the correct value of 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.
Riemann's prime-power counting function
Riemann's prime-power counting function is usually denoted as or It has jumps of at prime powers and it takes a value halfway between the two sides at the discontinuities of (x). That added detail is used because the function may then be defined by an inverse Mellin transform.
Formally, we may define by
where the variable in each sum ranges over all primes within the specified limits.
We may also write
where is the von Mangoldt function and
The Möbius inversion formula then gives
where is the Möbius function.
Knowing the relationship between the logarithm of the Riemann zeta function and the von Mangoldt function , and using the Perron formula we have
Chebyshev's function
The Chebyshev function weights primes or prime powers by :
For ,
and
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 the second Chebyshev function ψ:
where
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. The integral is equal to the series 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 log x), where Ei is the analytic continuation of the exponential integral function from negative reals to the complex plane with branch cut along the positive reals.
Thus, Möbius inversion formula gives us
valid for x > 1, where
is Riemann's R-function and is the Möbius function. The latter series for it is known as Gram series. Because for all , this series converges for all positive x by comparison with the series for . The logarithm in the Gram series of the sum over the non-trivial zero contribution should be evaluated as and not .
Folkmar Bornemann proved, when assuming the conjecture that all zeros of the Riemann zeta function are simple, that
where runs over the non-trivial zeros of the Riemann zeta function and .
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 a good estimator of for x > 1. In fact, since the second term approaches 0 as , while the amplitude of the "noisy" part is heuristically about estimating by alone is just as good, and fluctuations of the distribution of primes may be clearly represented with the function
Inequalities
Here are some useful inequalities for (x).
for x ≥ 17.
The left inequality holds for x ≥ 17 and the right inequality holds for x > 1. The constant 1.25506 is to 5 decimal places, as has its maximum value at x = 113.
Pierre Dusart proved in 2010:
for , and
for .
Here are some inequalities for the nth prime, pn. The upper bound is due to Rosser (1941), the lower one to Dusart (1999):
for n ≥ 6.
The left inequality holds for n ≥ 2 and the right inequality holds for n ≥ 6.
An approximation for the nth prime number is
Ramanujan proved that the inequality
holds for all sufficiently large values of .
In Dusart proved (Proposition 6.6) that, for ,
and (Proposition 6.7) that, for ,
More recently, Dusart
has proved (Theorem 5.1) that, for
and that, for ,
The Riemann hypothesis
The Riemann hypothesis implies a much tighter bound on the error in the estimate for , and hence to a more regular distribution of prime numbers,
Specifically,
See also
Foias constant
Bertrand's postulate
Oppermann's conjecture
Ramanujan prime
References
Notes
External links
Chris Caldwell, The Nth Prime Page at The Prime Pages.
Tomás Oliveira e Silva, Tables of prime-counting functions.
Analytic number theory
Prime numbers
Arithmetic functions