Fermat died with the belief that he had found a long-sought-for law of prime numbers in the formula 2 2 1 + 1 = a prime, but he admitted that he was unable to prove it rigorously. The law is not true, as was pointed out by Euler in the example 22 5 + 1 = 4,294,967,297 = 6,700,417 times 641. The American lightning calculator Zerah Colburn, when a boy, readily found the factors but was unable to explain the method by which he made his marvellous mental computation.

p. 180; Cited in: Moritz (1914, 157); Persons and anecdotes. - A History of Mathematics (1893)