@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/}
}