Definition:Brun's Constant

Definition
Brun's constant is the sum of the series consisting of the reciprocals of the twin primes:
 * $B_2 := \left({\dfrac 1 3 + \dfrac 1 5}\right) + \left({\dfrac 1 5 + \dfrac 1 7}\right) + \left({\dfrac 1 {11} + \dfrac 1 {13} }\right) + \left({\dfrac 1 {17} + \dfrac 1 {19} }\right) + \left({\dfrac 1 {29} + \dfrac 1 {31} }\right) + \cdots$

Its approxmiate decimal expansion is:
 * $B_2 \approx 1 \cdotp 90216 \, 05831 \, 04 \ldots$

Estimates of its value are occasionally refined as further work is done to establish its nature.

Also defined as
Some sources do not include one of the instances of $\dfrac 1 5$.

Also see

 * Brun's Theorem