For questions on prime twins.

A twin prime is a prime number that differs from another prime number by two, for example the twin prime pair $(41, 43)$. Sometimes the term twin prime is used for a pair of twin primes; an alternative name for this is prime twin or prime pair.

It is currently unknown whether or not there are infinitely many pairs of twin primes. However, it is known (Brun's theorem) that the sum of the inverses of the twin primes is finite (it is about $1.902$).