@article{AM2_1990__39_2-3_137345,
     author = {N. Abe and Y. Nakanishi and S. Yamagushi},
     title = {Circles and spheres in {pseudo-Riemannian} geometry.},
     journal = {Aequationes mathematicae},
     pages = {134--145},
     publisher = {mathdoc},
     volume = {39},
     number = {2-3},
     year = {1990},
     zbl = {0705.53026},
     url = {http://geodesic.mathdoc.fr/item/AM2_1990__39_2-3_137345/}
}