SMET, A.; VERSCHOREN, A. The strong (pde) condition. Quaestiones Mathematicae, [S. l.], v. 23, n. 4, p. 495–505, 2006. DOI: 10.2989/16073600009485992. Disponível em: https://www.ajol.info/index.php/qm/article/view/21849. Acesso em: 13 aug. 2026.