The abundancy of a number n is defined as the ratio σ(n)/n, where σ(n) is the divisor function. For n = 1, 2, ..., the first few values are 1, 3/2, 4/3, 7/4, 6/5, 2, 8/7, 15/8, 13/9, 9/5, 12/11, 7/3, 14/13, ... (OEIS A017665 and A017666). A positive integer n for which σ(n)/n is an integer is called a multiperfect number. The first few are 1, 6, 28, 120, 496, 672, 8128, ... (OEIS A007691), corresponding to the abundancies 1, 2, 2, 3, 2, 3, 2, 4, 4, ... (OEIS A054030).

abundance | abundant number | multiperfect number