Gorbachuk, M. L., Ya. I. Hrushka, and S. M. Torba. 2005. “Direct and Inverse Theorems in the Theory of Approximation by the Ritz Method”. Ukrains’kyi Matematychnyi Zhurnal 57 (5), 633–643. https://umj.imath.kiev.ua/index.php/umj/article/view/3629.