Quasi-copulas as linear combinations of copulas

We prove that every quasi-copula can be written as a uniformly converging infinite sum of multiples of copulas. Furthermore, we characterize those quasi-copulas which can be written as a finite sum of multiples of copulas, i.e., that are a linear combination of two copulas. This generalizes a recent result of Fernandez-Sanchez, Quesada-Molina, and ubeda-Flores who considered linear combinations of discrete copulas.