The convolution sum, is evaluated for a ∈ {0, 1, 2, 3} and all n ∈ ℕ. This completes the partial evaluation given in the paper of J. G. Huard, Z. M. Ou, B. K. Spearman, K. S. Williams.

Convolution sums, Sum of divisors function, Theta functions
dx.doi.org/10.1007/s10587-009-0052-0
Czechoslovak Mathematical Journal
School of Mathematics and Statistics

Alaca, A, Alaca, S, & Williams, K.S. (2009). Evaluation of the sums. Czechoslovak Mathematical Journal, 59(3), 847–859. doi:10.1007/s10587-009-0052-0