Gorbachuk, M. L., Hrushka, Y. I. and Torba, S. M. (2005) “Direct and Inverse Theorems in the Theory of Approximation by the Ritz Method”, Ukrains’kyi Matematychnyi Zhurnal, 57(5), pp. 633–643. Available at: https://umj.imath.kiev.ua/index.php/umj/article/view/3629 (Accessed: 28May2024).