1.902160578...

This number is neither prime nor composite.

+ In 1919, Brun showed that the sum of the reciprocals of the twin primes converges to a sum now called Brun's Constant: (1/3 + 1/5) + (1/5 + 1/7) + (1/11 + 1/13) + (1/17 + 1/19) + ....

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