ROFE-BEKETOV, F. S.; KHOLKIN, A. M. Sturm type oscillation theorems for equations with block-triangular matrix coefficients. Methods of Functional Analysis and Topology, [S. l.], v. 18, n. 2, p. 176–188, 2012. Disponível em: https://zen.imath.kiev.ua/index.php/mfat/article/view/513. Acesso em: 16 dec. 2025.