Evaluation of the convolution sums ∑a1m1+a2m2+a3m3+a4m4=nσ(m1)σ(m2)σ(m3)σ(m4) with lcm(a1,a2,a3,a4) ≤ 4

Abstract

The generating functions of divisor functions are quasimodular forms of weight 2 and the product of them is a quasimodular form of higher weight. In this work, we evaluate the convolution sums [Formula: see text] for the positive integers [Formula: see text], and [Formula: see text] with lcm[Formula: see text]. We reprove the known formulas for the number of representations of a positive integer [Formula: see text] by each of the quadratic forms [Formula: see text] as an application of the new identities proved in this paper.