Series in biorthogonal systems of functions and their application to the solution of boundary-value problems of the Helmholtz equation. Neliniini Kolyvannya, [S. l.], v. 27, n. 1, p. 31–42, 2024. DOI: 10.3842/nosc.v27i1.1451. Disponível em: https://twinhead.imath.kiev.ua/index.php/nosc/article/view/1127. Acesso em: 8 dec. 2025.