Gorbachuk ML, Hrushka YI, Torba SM. Direct and Inverse Theorems in the Theory of Approximation by the Ritz Method. Ukr. Mat. Zhurn. [Internet]. 2005May25 [cited 2024May8];57(5):633–643. Available from: https://umj.imath.kiev.ua/index.php/umj/article/view/3629