Named for Nicomachus of Gerasa, first-century arithmetician and numerologist who has been credited with the discovery that 496 and 8128 are the third and fourth perfect numbers, i.e., numbers that are the sums of their factors (including unity but excluding the numbers themselves). The perfection of 6 and 28 was known to even earlier Greek and Hindu mathematicians. In 1644, Mersenne showed that the sequence of perfect numbers is given by the product of 2ⁿ-1 and 2^n-1, where 2ⁿ-1 is prime---as then is n, which takes the values 2, 3, 5 and 7 for the first four cases. Name proposed by the second discoverer, following a suggestion by B. G. Marsden, who wrote the citation.