Brun's constant
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Brun's constant $B_2$ is the sum of the reciprocals of the pairs of twin primes, i.e. $$B_2 = \displaystyle\sum_{p,p+2 \mathrm{\hspace{2pt} prime}} \dfrac{1}{p} + \dfrac{1}{p+2}=1.9021605831\ldots.$$
References
Brun's constant
La série 1/5+1/7+1/11+1/13+1/17+1/19+1/29+1/31+1/41+1/43+1/59+1/61+..., où les dénominateurs sont nombres premiers jumeaux est convergente ou finie
