The function sigma(n) will compute the sum of the positive divisors of n.

The function sigma[k](n)  will compute the sum of the kth power of positive divisors of n.

This function is part of the numtheory package, and so can be used in the form sigma[..](..) only after performing the command with(numtheory) or with(numtheory, sigma).  The function sigma(..) can always be accessed in the above form or the long form numtheory[sigma](..).