From: Convolution identities for twisted Eisenstein series and twisted divisor functions
N : prime
∑ k = 1 N − 1 σ 1 ∗ (k) σ 1 ∗ (N−k)
∑ k = 1 N − 1 σ 1 ∗ (k;4) σ 1 ∗ (N−k;4)
N = 2q + 1
2 q 2 (q+1)
3 q 2 (q+1)