AUTHOR : Torrisi M. 1. ааа Differential Invariants for a Class of Nonlinear Wave Equations . M.Torrisi, A.Valenti (Dipartimento di Matematica e Informatica, University of Catania, Catania, Italy); N.H.Ibragimov (Research Centre ALGA: Advances in Lie Group Analysis, Department of Health, Science and Mathematics, Blekinge Institute of Technology, Karlskrona, Sweden) . ... M.Torrisi (Department of Mathematics and Informatics, University of Catania, Catania, Italy) . ...