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 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:
Here γ is the famous Euler–Mascheroni constant, which has a similar definition involving a sum over all integers (not just the primes).
The value of M is approximately
Mertens' 2nd theorem says 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.
The number was used as a bid in the Nortel patent auction. The bid posted by Google was one of three that were based on mathematical numbers.[1]