Vassilev, Vassil. “GROUP PROPERTIES OF A CLASS OF FOURTH-ORDER PARTIAL DIFFERENTIAL EQUATIONS”. Ann. Sofia Univ. Fac. Math. and Inf. 82, no. 2 (December 12, 1991): 163–178. Accessed December 5, 2024. https://ftl5.uni-sofia.bg/index.php/fmi/article/view/542.