BLIZNIKAS, V. Über Geometrie des Systems der partiellen Differentialgleichungen zweiter Ordnung. Lithuanian Mathematical Journal, [S. l.], v. 7, n. 2, p. 249–264, 1967. DOI: 10.15388/LMJ.1967.19953. Disponível em: https://test.zurnalai.vu.lt/LMJ/article/view/19953. Acesso em: 29 oct. 2025.