HEIKKILÄ, S. On the complete integrability of the first order total differential equation. Annales Fennici Mathematici, [S. l.], n. 495, 1971. DOI: 10.5186/aasfm.1971.495. Disponível em: https://afm.journal.fi/article/view/134151. Acesso em: 26 nov. 2024.