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This category contains definitions related to Abundancy.
Related results can be found in Category:Abundancy.

Let $n$ be a positive integer.

Let $\map \sigma n$ be the sigma function of $n$.

That is, let $\map \sigma n$ be the sum of all positive divisors of $n$.

Then the abundancy index of $n$ is defined as $\dfrac {\map \sigma n} n$.