perfect number

《初等数论》 · 第 62 页

一个正整数 $m$ 称为是完全数，如果 $\sigma(m)=2m$。

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《Introduction to Analytic Number Theory》 · 第 4 页 · 历史导言

The number 6 was called a perfect number because 6 is the sum of all its proper divisors, that is, the sum of all divisors less than 6.