U. D. Bekbaev, “Equivalence and invariants of second-order linear partial differential equations in two variables under a change of variables”, Izv. Vyssh. Uchebn. Zaved. Mat., 1999, no. 11, 3–10; Russian Math. (Iz. VUZ), 43:11 (1999), 1–8
U. D. Bekbaev, “On the equivalence and differential rational invariants of second-order linear partial differential equations of two variables under a change of variables”, Differ. Uravn., 34:6 (1998), 835–837; Differ. Equ., 34:6 (1998), 833–836
1994
3.
U. D. Bekbaev, “On equivalence and invariants of ordinary linear differential equations”, Differ. Uravn., 30:5 (1994), 911–912; Differ. Equ., 30:5 (1994), 846–847