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In mathematics, a harmonic divisor number, or Ore number (named after Øystein Ore who defined it in 1948), is a positive integer whose divisors have a harmonic mean that is an integer. The first few harmonic divisor numbers are

1, 6, 28, 140, 270, 496, 672, 1638, 2970, 6200, 8128, 8190 (sequence A001599 in OEIS).

For example, the harmonic divisor number 6 has the four divisors 1, 2, 3, and 6. Their harmonic mean is an integer:

$$\frac{4}{\frac{1}{1}+\frac{1}{2}+\frac{1}{3}+\frac{1}{6}}=2.$$

The number 140 has divisors 1, 2, 4, 5, 7, 10, 14, 20, 28, 35, 70, and 140. Their harmonic mean is:

$$\frac{12}{\frac{1}{1}+\frac{1}{2}+\frac{1}{4}+\frac{1}{5}+\frac{1}{7}+\frac{1}{10} +\frac{1}{14}+\frac{1}{20}+\frac{1}{28}+\frac{1}{35}+\frac{1}{70}+\frac{1}{140}}=5$$

5 is an integer, making 140 a harmonic divisor number.
Harmonic divisor numbers and perfect numbers

For any integer M, as Ore observed, the product of the harmonic mean and arithmetic mean of its divisors equals M itself; as is obvious from the definition. Therefore, M is harmonic, with harmonic mean of divisors k, if and only if the average of its divisors is the product of M with a unit fraction 1/k.

Ore showed that every perfect number is harmonic. To see this, observe that the sum of the divisors of a perfect number M is exactly 2M; therefore, the average of the divisors is M(2/τ(M)), where τ(M) denotes the number of divisors of M. For any M, τ(M) is odd if and only if M is a square number, for otherwise each divisor d of M can be paired with a different divisor M/d. But, no perfect number can be a square: this follows from the known form of even perfect numbers and from the fact that odd perfect numbers (if they exist) must have a factor of the form qα where α ≡ 1 (mod 4). Therefore, for a perfect number M, τ(M) is even and the average of the divisors is the product of M with the unit fraction 2/τ(M); thus, M is a harmonic divisor number.

Ore conjectured that no odd harmonic divisor numbers exist other than 1. If the conjecture is true, this would imply the nonexistence of odd perfect numbers.
Bounds and computer searches

W. H. Mills (unpublished; see Muskat) showed that any odd harmonic divisor number above 1 must have a prime power factor greater than 107, and Cohen showed that any such number must have at least three different prime factors. Cohen and Sorli (2010) showed that there are no odd harmonic divisor numbers smaller than 1024.

Cohen, Goto, and others starting with Ore himself have performed computer searches listing all small harmonic divisor numbers. From these results, lists are known of all harmonic divisor numbers up to 2×109, and all harmonic divisor numbers for which the harmonic mean of the divisors is at most 300.
References

Bogomolny, Alexander. "An Identity Concerning Averages of Divisors of a Given Integer". Retrieved 2006-09-10.

Cohen, Graeme L. (1997). "Numbers Whose Positive Divisors Have Small Integral Harmonic Mean". Mathematics of Computation 66 (218): 883–891. doi:10.1090/S0025-5718-97-00819-3.

Cohen, Graeme L.; Sorli, Ronald M. (2010). "Odd harmonic numbers exceed 1024". Mathematics of Computation 79 (272): 2451. doi:10.1090/S0025-5718-10-02337-9. ISSN 0025-5718. Posted electronically on April 9, 2010; to appear in print.

Goto, Takeshi. "(Ore's) Harmonic Numbers". Retrieved 2006-09-10.

Muskat, Joseph B. (1966). "On Divisors of Odd Perfect Numbers". Mathematics of Computation (American Mathematical Society) 20 (93): 141–144. doi:10.2307/2004277. JSTOR 2004277.

Ore, Øystein (1948). "On the averages of the divisors of a number". American Mathematical Monthly (Mathematical Association of America) 55 (10): 615–619. doi:10.2307/2305616. JSTOR 2305616.

Weisstein, Eric W., "Harmonic Divisor Number" from MathWorld.

Mathematics Encyclopedia