@article{GDZPPN002489775, title = {The View-Obstruction Problem for Spheres in ...5.}, journal = {Monatshefte f\"ur Mathematik}, volume = {121}, year = {1996}, pages = {21-34}, zbl = {0856.11033}, url = {http://dml.mathdoc.fr/item/GDZPPN002489775} }
Wilker, J.B.; Dumir, V.C.; Hans-Gill, R.J. The View-Obstruction Problem for Spheres in ...5.. Monatshefte für Mathematik, Tome 121 (1996) pp. 21-34. http://gdmltest.u-ga.fr/item/GDZPPN002489775/