# Meissel–Mertens constant

In the limit, the sum of the reciprocals of the primes < n and the function ln(ln(n)) are separated by a constant, the Meissel–Mertens constant (labelled M above).

The Meissel–Mertens constant (named after Ernst Meissel and Franz Mertens), also referred to as Mertens constant, Kronecker's constant, Hadamardde 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:

$M = \lim_{n \rightarrow \infty } \left( \sum_{p \leq n} \frac{1}{p} - \ln(\ln(n)) \right)=\gamma + \sum_{p} \left[ \ln\! \left( 1 - \frac{1}{p} \right) + \frac{1}{p} \right].$

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

The plot of the prime harmonic sum up to $n=2^{15}, 2^{16}, \ldots, 2^{46}=7.04\times 10^{13}$ and the Merten's approximation to it. The original of this figure has y axis of the length 8 cm and spans the interval (2.5, 3.8), so if the n axis would be plotted in the linear scale instead of logarithmic, then it should be $5.33(3)\times 10^9$ km long --- that is the size of the Solar System.

The value of M is approximately

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

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.

## Trivia

The Meisel-Mertens constant was used by Google when bidding in the Nortel patent auction. Google posted three bids based on mathematical numbers: $1,902,160,540 (Brun's constant),$2,614,972,128 (Meissel–Mertens constant), and \$3.14159 billion (π).[1]