On the types of linear partial differential equations of the second order in three independent variables which are unaltered by the transformations of a continuous group
Author: J. E. Campbell
Journal: Trans. Amer. Math. Soc. 1 (1900), 243-258
MSC: Primary 35A30; Secondary 22E15
Erratum: Trans. Amer. Math. Soc. 1 (1900), 509.
MathSciNet review: 1500535