GorbachukM. L., HrushkaYa. I., and TorbaS. M. “Direct and Inverse Theorems in the Theory of Approximation by the Ritz Method”. Ukrains’kyi Matematychnyi Zhurnal 57, no. 5 (May 25, 2005): 633–643. Accessed July 27, 2024. https://umj.imath.kiev.ua/index.php/umj/article/view/3629.