VASSILEV, V. GROUP PROPERTIES OF A CLASS OF FOURTH-ORDER PARTIAL DIFFERENTIAL EQUATIONS. Ann. Sofia Univ. Fac. Math. and Inf., [S. l.], v. 82, n. 2, p. 163–178, 1991. Disponível em: https://admin.uni-sofia.bg/index.php/fmi/article/view/542. Acesso em: 16 jan. 2025.