thumb|300px|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 the Mertens constant, Kronecker's constant (after Leopold Kronecker), Hadamard–de la Vallée-Poussin constant (after Jacques Hadamard and Charles Jean de la Vallée-Poussin), 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:
:<math>M = \lim_{n \rightarrow \infty } \left(
\sum_{\scriptstyle p\text{ prime}\atop \scriptstyle p\le n} \frac{1}{p} - \ln(\ln n) \right)=\gamma + \sum_{p} \left[ \ln\! \left( 1 - \frac{1}{p} \right) + \frac{1}{p} \right].</math>
Here γ is the Euler–Mascheroni constant, which has an analogous definition involving a sum over all integers (not just the primes).
thumb|300px| The plot of the prime harmonic sum up to <math>n=2^{15}, 2^{16}, \ldots, 2^{46} \approx 7.04 \times 10^{13}</math> and the Merten's approximation to it. If the n axis were plotted in the linear scale instead of logarithmic, then the figure would be <math>5.33(3) \times 10^9</math> km long —approximately the distance to Neptune.
The value of M is approximately
:M ≈ 0.2614972128476427837554268386086958590516... .
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.
In popular culture
The Meissel-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).
See also
- Divergence of the sum of the reciprocals of the primes
- Prime zeta function