@article{oai:tsukuba.repo.nii.ac.jp:00016110,
 author = {Wakabayashi, Seiichiro},
 journal = {Tsukuba Journal of Mathematics},
 month = {Dec},
 pages = {91--107},
 title = {Propagation of singularities for hyperbolic operators with constant coefficient principal part},
 volume = {2},
 year = {1978}
}