On Invariants of Second Order Linear Partial Differential Equations in Two Variables
1999 ◽
Vol 44
(2)
◽
pp. 177-203
◽
2004 ◽
Vol 152
(1)
◽
pp. 29-41
◽
2015 ◽
pp. 49-98
1980 ◽
Vol 29
(4)
◽
pp. 454-461
2002 ◽
pp. 213-224
2020 ◽
pp. 49-99