Harmonic divisor number
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A harmonic divisor number, or Ore number, is a number whose divisors, averaged in a harmonic mean, results in an integer. The first few harmonic divisor numbers are
1, 6, 28, 140, 270, 496, 672, 1638, 2970, 6200, 8128, 8190
Three of these listed are also perfect numbers, and like perfect numbers, harmonic divisor numbers tend to be even numbers, at least in the range observed. In 1972, W.H. Mills proved that, besides 1, there are no odd harmonic divisor numbers with prime power factors less than 107.
For example, 496 is a harmonic divisor number because 10, its number of divisors, divided by the sum of the reciprocals of its divisors, 1, 2, 4, 8, 16, 31, 62, 124, 248 and 496, (the harmonic mean), yields an integer, 5 in this case.