A perfect number is one that is exactly equal to the sum of its proper divisors (again, excluding itself). The first perfect number is 6, as its divisors (1, 2, 3) all up to 6. Six is followed by 28, 496, and 8,128. Early Greek mathematicians knew only of these first 4 perfect numbers; Nichomatus discovered 8,128 by the year A.D. 100. Three more were discovered, the first circa 1456 (33,550,336) by an unknown mathematician, and in 1588 (8,589,869,056 and 137,438,691,328) by Italian mathematician Pietro Cataldi in 1588. --
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