CALÁBEK, Pavel. Second-order differential systems and a regularization operator. Annales Mathematicae Silesianae, [S. l.], v. 10, p. 67–78, 1996. Disponível em: https://trrest.vot.pl/ojsus/index.php/AMSIL/article/view/14193. Acesso em: 7 may. 2025.