@article{oai:tsukuba.repo.nii.ac.jp:00017340,
 author = {Nakazawa, Nobuo},
 issue = {2},
 journal = {Tsukuba Journal of Mathematics},
 month = {Dec},
 pages = {257--278},
 title = {On hypoellipticity for a class of pseudo-differential operators},
 volume = {24},
 year = {2000}
}