@article{oai:tsukuba.repo.nii.ac.jp:00016047,
 author = {Kwon, K. H.},
 issue = {2},
 journal = {Tsukuba Journal of Mathematics},
 month = {Dec},
 pages = {443--451},
 title = {Hypoellipticity of systems of pseudo differential operators with double characteristics},
 volume = {13},
 year = {1989}
}