@article{oai:tsukuba.repo.nii.ac.jp:00015785,
 author = {Morimoto, Yoshinori},
 issue = {1},
 journal = {Tsukuba Journal of Mathematics},
 month = {Jun},
 pages = {187--200},
 title = {Hypoelliptic operators of principal type with infinite degeneracy},
 volume = {19},
 year = {1995}
}