@article{GDZPPN002307480, title = {Moebius Planes with Locally Euclidean Circles Are Flat.}, journal = {Mathematische Annalen}, volume = {201}, year = {1973}, pages = {149-156}, zbl = {0234.50021}, url = {http://dml.mathdoc.fr/item/GDZPPN002307480} }
Groh, Hansjoachim. Moebius Planes with Locally Euclidean Circles Are Flat.. Mathematische Annalen, Tome 201 (1973) pp. 149-156. http://gdmltest.u-ga.fr/item/GDZPPN002307480/