@article{10_1017_S0017089599970969,
     author = {Defever, F. and Deszcz, R. and Sent\"urk, Z. and Verstraelen, L. and Yaprak, S.},
     title = {P.J. {Ryan's} problem in {semi-Riemannian} space forms},
     journal = {Glasgow mathematical journal},
     pages = {271--281},
     year = {1999},
     volume = {41},
     number = {2},
     doi = {10.1017/S0017089599970969},
     url = {http://geodesic.mathdoc.fr/articles/10.1017/S0017089599970969/}
}