KHRABUSTOVSKYI, V. Expansion in eigenfunctions of relations generated by pair of operator differential expressions. Methods of Functional Analysis and Topology, [S. l.], v. 15, n. 2, p. 137–151, 2009. Disponível em: https://zen.imath.kiev.ua/index.php/mfat/article/view/414. Acesso em: 16 dec. 2025.