LEVCHUK, V. M. On conditionless bases of the kernels generated by differential equations of the second order. Ukrains’kyi Matematychnyi Zhurnal, [S. l.], v. 69, n. 3, p. 332–347, 2017. Disponível em: https://umj.imath.kiev.ua/index.php/umj/article/view/1700. Acesso em: 5 dec. 2025.