Sigma function

This page is about the sigma function that calculates the sum of the factors of a number. To see the page which discusses Rado's sigma function, see busy beaver.

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In number theory, the sigma function (not to be confused with summation) is the function that finds the sum of the positive divisors of a number, denoted by σ(n).

It is a multiplicative function. If p is a prime number, σ(p) is p + 1 because, by definition, the factors of a prime number are 1 and itself.