 There is also the double Dirichlet series

σ"k"("n") is the divisor function (i.e.the sum of the "k"-th powers of the divisors of "n", including 1 and "n").σ0("n"), the number of divisors of "n", is usually written "d"("n") and σ1("n"), the sum of the divisors of "n", is usually written σ("n").