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@article{JRAM_1931__164_149746, author = {L.J. Mordell}, title = {The condition for integer solutions of ax2 + by2 + cz2 + dt2 = 0.}, journal = {Journal f\"ur die reine und angewandte Mathematik}, pages = {40--49}, publisher = {mathdoc}, volume = {164}, year = {1931}, url = {http://geodesic.mathdoc.fr/item/JRAM_1931__164_149746/} }
L.J. Mordell. The condition for integer solutions of ax2 + by2 + cz2 + dt2 = 0.. Journal für die reine und angewandte Mathematik, Tome 164 (1931), pp. 40-49. http://geodesic.mathdoc.fr/item/JRAM_1931__164_149746/