Английская Википедия:Harmonic divisor number
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In mathematics, a harmonic divisor number or Ore number is a positive integer whose divisors have a harmonic mean that is an integer. The first few harmonic divisor numbers are
Harmonic divisor numbers were introduced by Øystein Ore, who showed that every perfect number is a harmonic divisor number and conjectured that there are no odd harmonic divisor numbers other than 1.
Examples
The number 6 has the four divisors 1, 2, 3, and 6. Their harmonic mean is an integer: <math display="block"> \frac{4}{\frac{1}{1}+\frac{1}{2}+\frac{1}{3}+\frac{1}{6}}=2.</math> Thus 6 is a harmonic divisor number. Similarly, the number 140 has divisors 1, 2, 4, 5, 7, 10, 14, 20, 28, 35, 70, and 140. Their harmonic mean is <math display="block">
\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. </math> Since 5 is an integer, 140 is a harmonic divisor number.
Factorization of the harmonic mean
The harmonic mean Шаблон:Math of the divisors of any number Шаблон:Mvar can be expressed as the formula <math display="block">H(n) = \frac{n\sigma_0(n)}{\sigma_1(n)}</math> where Шаблон:Math is the [[divisor function|sum of Шаблон:Mvarth powers of the divisors]] of Шаблон:Mvar: Шаблон:Math is the number of divisors, and Шаблон:Math is the sum of divisors Шаблон:Harv. All of the terms in this formula are multiplicative but not completely multiplicative. Therefore, the harmonic mean Шаблон:Math is also multiplicative. This means that, for any positive integer Шаблон:Mvar, the harmonic mean Шаблон:Math can be expressed as the product of the harmonic means of the prime powers in the factorization of Шаблон:Mvar.
For instance, we have <math display="block">H(4) = \frac{3}{1+\frac{1}{2}+\frac{1}{4}}=\frac{12}7,</math> <math display="block">H(5) = \frac{2}{1+\frac{1}{5}} = \frac53,</math> <math display="block">H(7) = \frac{2}{1+\frac{1}{7}} = \frac74,</math> and <math display="block">H(140) = H(4 \cdot 5 \cdot 7) = H(4)\cdot H(5)\cdot H(7) = \frac{12}{7}\cdot \frac{5}{3}\cdot \frac{7}{4} = 5.</math>
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 can be seen from the definitions. 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. Шаблон:Harvtxt 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
- Шаблон:Cite web
- Шаблон:Cite journal
- Шаблон:Cite journal
- Шаблон:Cite web
- Шаблон:Cite book
- Шаблон:Cite journal
- Шаблон:Cite journal
- Шаблон:MathWorld
Шаблон:Divisor classes Шаблон:Classes of natural numbers
