In number theory, the Poussin proof is the proof of an identity related to the fractional part of a ratio.
In 1838, Peter Gustav Lejeune Dirichlet proved an approximate formula for the average number of divisors of all the numbers from 1 to η:
where d represents the divisor function, and γ represents the Euler-Mascheroni constant.
In 1898, Charles Jean de la Vallée-Poussin proved that if a large number η is divided by all the primes up to η, then the average fraction by which the quotient falls short of the next whole number is γ:
where {x} represents the fractional part of x, and π represents the prime-counting function. For example, if we divide 29 by 2, we get 14.5, which falls short of 15 by 0.5.
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