Meissel–Mertens constant

The Meissel–Mertens constant (named after Ernst Meissel and Franz Mertens), also referred to as Mertens constant, Kronecker's constant, Hadamard–de la Vallée-Poussin constant or the prime reciprocal constant, is a mathematical constant in number theory, defined as the limiting difference between the harmonic series summed only over the primes and the natural logarithm of the natural logarithm:

M = lim n → ∞ ( ∑ p ≤ n 1 p − ln ⁡ ( ln ⁡ ( n ) ) ) = γ + ∑ p [ ln ( 1 − 1 p ) + 1 p ] .

Here γ is the Euler–Mascheroni constant, which has an analogous definition involving a sum over all integers (not just the primes).

The value of M is approximately

M ≈ 0.2614972128476427837554268386086958590516... (sequence A077761 in the OEIS).

Mertens' second theorem establishes that the limit exists.

The fact that there are two logarithms (log of a log) in the limit for the Meissel–Mertens constant may be thought of as a consequence of the combination of the prime number theorem and the limit of the Euler–Mascheroni constant.