Brun's Theorem/Historical Note

Historical Note on Brun's Theorem
While Brun's Theorem proves that the sum of the reciprocals of all the twin primes converges, it has not yet been established what it actually converges to.

The number, whatever it is, is referred to as Brun's constant. Its best estimate is updated as and when the occasion demands.

As pointed out by in his  of $1997$:
 * Its value is exceedingly hard to calculate.