GOMILKO, A. M. Expansion of a bundle of fourth-order differential operators in a part of its eigenfunctions. Ukrains’kyi Matematychnyi Zhurnal, [S. l.], v. 45, n. 12, p. 1601–1612, 1993. Disponível em: https://umj.imath.kiev.ua/index.php/umj/article/view/5967. Acesso em: 5 dec. 2025.