Gorbachuk, M. L., et al. “Direct and Inverse Theorems in the Theory of Approximation by the Ritz Method”. Ukrains’kyi Matematychnyi Zhurnal, vol. 57, no. 5, May 2005, pp. 633–643, https://umj.imath.kiev.ua/index.php/umj/article/view/3629.