@article{oai:tsukuba.repo.nii.ac.jp:00016033, author = {Suzuki, Michiharu}, issue = {1}, journal = {Tsukuba Journal of Mathematics}, month = {Jun}, pages = {217--234}, title = {Hypoellipticity for a class of degenerate elliptic operators of second order}, volume = {16}, year = {1992} }