Abdulkasim Akhmedov, Mohd Zuki Salleh, and Abdumalik Rakhimov. “The Approximation of the Solution of Wave Problems by Spectral Expansions Connected With Elliptic Differential Operators”. Journal of Advanced Research in Fluid Mechanics and Thermal Sciences, vol. 86, no. 2, Mar. 2024, pp. 101-6, https://semarakilmu.com.my/journals/index.php/fluid_mechanics_thermal_sciences/article/view/8147.