FARDIGOLA, L. V. Transformation operators in controllability problems for the degenerate wave equation with variable coefficients. Ukrains’kyi Matematychnyi Zhurnal, [S. l.], v. 70, n. 8, p. 1128–1142, 2018. Disponível em: https://umj.imath.kiev.ua/index.php/umj/article/view/1623. Acesso em: 5 dec. 2025.