“…Remark also that any equation which is equivalent to (12) belongs to the class H 1,1 (when k ≡ h ≡ 0) or H 2,2 (when k = 0 and h = 0). All equations of the class H 1,1 are locally equivalent to the wave equation v xy = 0 [26]. Therefore, we shall consider only equations of the class H 2,2 .…”