KOVALEV, Yu. G. Point interactions on the line and Riesz bases of δ -functions. Ukrains’kyi Matematychnyi Zhurnal, [S. l.], v. 69, n. 12, p. 1615–1624, 2017. Disponível em: https://umj.imath.kiev.ua/index.php/umj/article/view/1808. Acesso em: 5 dec. 2025.