KOSHMANENKO, V. D.; TUHAI, H. V. On the structure of resolvent of singularly perturbed operator solving the problem of eigenvalues. Ukrains’kyi Matematychnyi Zhurnal, [S. l.], v. 56, n. 9, p. 1292–1297, 2004. Disponível em: https://umj.imath.kiev.ua/index.php/umj/article/view/3843. Acesso em: 6 dec. 2025.