2305843009213693951

This number is a prime.

                                                        230584300 9213693951

Wonder Book of Strange Facts (Ripley's Believe It or Not, Inc., 1957, p. 100) states that this (the 9th Mersenne prime) is the number of ways to make change for a five-dollar bill using pennies, nickels, dimes, quarters, half-dollar, and dollar coins. The correct answer is 98411, making this one of the largest prime errors ever found. [Rupinski]

Russian priest and mathematician Ivan Mikheevich Pervushin discovered in 1883 that Marin Mersenne's famous list was mistaken to exclude 2⁶¹-1 = 2305843009213693951. It was known for a while as "Pervushin's number."