@article{1183425280, author = {Bell, E. T.}, title = {The equation $ds^2 = dx^2 + dy^2 + dz^2$}, journal = {Bull. Amer. Math. Soc.}, volume = {27}, number = {3}, year = {1920}, pages = { 312-318}, language = {en}, url = {http://dml.mathdoc.fr/item/1183425280} }
Bell, E. T. The equation $ds^2 = dx^2 + dy^2 + dz^2$. Bull. Amer. Math. Soc., Tome 27 (1920) no. 3, pp. 312-318. http://gdmltest.u-ga.fr/item/1183425280/