Pervushin's number

Pervushin's Number is 2,305,843,009,213,693,951. It's the ninth mersenne prime and can be expressed compactly as 2^61-1. It is traditionally denoted as M(61).

This number was first proven to be prime by Ivan Mikheevich Pervushin in November of 1883 (hence the designation of Pervushin's Number). At the time of it's discovery it was the 2nd largest known prime. It remained as such until 1911.

It is an example of large numbers in number theory.