@article{oai:niigata-u.repo.nii.ac.jp:00002191, author = {Innami, Nobuhiro}, issue = {1}, journal = {Nihonkai Mathematical Journal, Nihonkai Mathematical Journal}, month = {}, pages = {5--23}, title = {Generalized Metrics Second Order Equations Satisfying Huygens' Principle}, volume = {6}, year = {1995} }