Closeness and normal solvability of the operator generated by a degenerate linear differential equation with variable coefficients. Neliniini Kolyvannya, [S. l.], v. 10, n. 4, p. 464–480, 2007. Disponível em: https://twinhead.imath.kiev.ua/index.php/nosc/article/view/451. Acesso em: 8 dec. 2025.