Variational principles for differential equations with symmetries and conservation laws. I. Second order scalar equations. Ian M. Anderson; Juha Pohjanpelto — 1994 Mathematische Annalen
Variational principles for differential equations with symmetries and conservation laws. II. Polynomial differential equations. Ian M. Anderson; Juha Pohjanpelto — 1995 Mathematische Annalen
Generalized symmetries of massless free fields on Minkowski space. Pohjanpelto, Juha; Anco, Stephen C. — 2008 SIGMA. Symmetry, Integrability and Geometry: Methods and Applications [electronic only]