Brun's constant
From specialfunctionswiki
Brun's constant $B_2$ is the sum of the reciprocals of the pairs of twin primes, i.e. $$B_2 = \displaystyle\sum_{p,p+2 \mathrm{\hspace{2pt} prime}} \dfrac{1}{p} + \dfrac{1}{p+2}=1.9021605831\ldots.$$
