Nomirovs'kii, D. A. “Convergence of the Galerkin Method for a Wave Equation With Singular Right-Hand Side”. Ukrains’kyi Matematychnyi Zhurnal, vol. 58, no. 6, June 2006, pp. 778–786, https://umj.imath.kiev.ua/index.php/umj/article/view/3495.