KATS, I. S. Power moments of negative order for the principal spectral function of a string. Ukrains’kyi Matematychnyi Zhurnal, [S. l.], v. 48, n. 9, p. 1209–1222, 1996. Disponível em: https://umj.imath.kiev.ua/index.php/umj/article/view/5236. Acesso em: 5 dec. 2025.