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In mathematics, the prime-counting function is the function counting the number of prime numbers less than or equal to some real number x. [1] [2] It is denoted by π(x) (unrelated to the number π). A symmetric variant seen sometimes is π 0 (x), which is equal to π(x) − 1 ⁄ 2 if x is exactly a prime number, and equal to π(x) otherwise.
The first such distribution found is π(N) ~ N / log(N) , where π(N) is the prime-counting function (the number of primes less than or equal to N) and log(N) is the natural logarithm of N. This means that for large enough N, the probability that a random integer not greater than N is prime is very close to 1 / log(N).
Riemann's original use of the explicit formula was to give an exact formula for the number of primes less than a given number. To do this, take F(log(y)) to be y 1/2 /log(y) for 0 ≤ y ≤ x and 0 elsewhere. Then the main term of the sum on the right is the number of primes less than x.
Let N be a positive integer, and let k be the number of primes less than or equal to N. Call those primes p 1, ... , p k. Any positive integer a which is less than or equal to N can then be written in the form = (), where each e i is either 0 or 1.
More precisely, they showed that there exist positive constants c and C such that for all sufficiently large numbers N, every even number less than N is the sum of two primes, with at most CN 1 − c exceptions. In particular, the set of even integers that are not the sum of two primes has density zero.
There exists a natural number N such that every even integer n larger than N is a sum of a prime less than or equal to n 0.95 and a number with at most two prime factors. Tomohiro Yamada claimed a proof of the following explicit version of Chen's theorem in 2015: [ 7 ]
Euler's totient function φ(n) is the number of positive integers less than n and coprime to n. Ramanujan defines a generalization of it, if = is the prime factorization of n, and s is a complex number, let
where () is the prime-counting function, equal to the number of primes less than or equal to x. The converse of this result is the definition of Ramanujan primes: The n th Ramanujan prime is the least integer R n for which π ( x ) − π ( x / 2 ) ≥ n , {\displaystyle \pi (x)-\pi (x/2)\geq n,} for all x ≥ R n . [ 2 ]