@article{oai:tsukuba.repo.nii.ac.jp:00017201,
 author = {Dencker, Nils and Morimoto, Yoshinori and Morioka, Tatsushi},
 issue = {2},
 journal = {Tsukuba Journal of Mathematics},
 month = {Oct},
 pages = {215--224},
 title = {Hypoellipticity for operators of infinitely degenerate Egorov type},
 volume = {23},
 year = {1999}
}