2305843009213693951
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The number 2305843009213693951 is the ninth Mersenne prime and is equal to 261 − 1. It was determined to be prime in 1883 by Ivan Mikheevich Pervushin and for this reason it is sometimes called Pervushin's number. At the time of Pervushin's proof it was the second-largest known prime number, (Édouard Lucas having shown earlier that 2127 − 1 is also prime), and it remained so until 1911.