|an integer, such as 28, that is equal to the sum of all its possible factors, excluding itself|
a positive integer that is equal to the sum of its proper divisors. The smallest perfect number is 6, which is the sum of 1, 2, and 3. Other perfect numbers are 28, 496, and 8,128. The discovery of such numbers is lost in prehistory. It is known, however, that the Pythagoreans (founded c. 525 BC) studied perfect numbers for their "mystical" properties.
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