@article{oai:tsukuba.repo.nii.ac.jp:00017310,
 author = {Tsukada, Nobutaka},
 issue = {2},
 journal = {Tsukuba Journal of Mathematics},
 month = {Oct},
 pages = {225--228},
 title = {An ultrapower which does not preserve the truth of a Π_2 sentence},
 volume = {23},
 year = {1999}
}