# Arithmetic number

2010 Mathematics Subject Classification: *Primary:* 11A [MSN][ZBL]

An integer for which the arithmetic mean of its positive divisors, is an integer. The first numbers in the sequence are 1, 3, 5, 6, 7, 11, 13, 14, 15, 17, 19, 20 . It is known that the natural density of such numbers is 1:[Guy (2004) p.76] indeed, the proportion of numbers less than $X$ which are not arithmetic is asymptotically[Bateman et al (1981)] $$ \exp\left( { -c \sqrt{\log\log X} } \right) $$ where $c = 2\sqrt{\log 2} + o(1)$.

A number $N$ is arithmetic if the number of divisors $\tau(N)$ divides the sum of divisors $\sigma(N)$. The natural density of integers $N$ for which $d(N)^2$ divides $\sigma(N)$ is 1/2.

## References

- Bateman, Paul T.; Erdős, Paul; Pomerance, Carl; Straus, E.G. "The arithmetic mean of the divisors of an integer". In Knopp, M.I..
*Analytic number theory, Proc. Conf., Temple Univ., 1980*. Lecture Notes in Mathematics**899**Springer-Verlag (1981) pp. 197–220. Zbl 0478.10027 - Guy, Richard K.
*Unsolved problems in number theory*(3rd ed.). Springer-Verlag (2004). ISBN 978-0-387-20860-2 Zbl 1058.11001. Section B2.

**How to Cite This Entry:**

Arithmetic number.

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