PIVOVARCHIK, V. M. On minimum number of distinct eigenvalues for a Stieltjes string problem on a tree. Ukrains’kyi Matematychnyi Zhurnal, [S. l.], v. 72, n. 1, p. 135–141, 2020. Disponível em: https://umj.imath.kiev.ua/index.php/umj/article/view/959. Acesso em: 5 dec. 2025.