Bogolyubov, A.N., M.D. Malykh, and A.A. Panin. 2009. “Two-Sided Estimates for the Eigenvalues of the Laplace Operator With Dirichlet Boundary Conditions and Their Application to Problems in the Mathematical Theory of Waveguides”. Numerical Methods and Programming (Vychislitel’nye Metody I Programmirovanie) 10 (19):83-93. https://en.num-meth.ru/index.php/journal/article/view/350.