@article{10_4064_ap84_2_5,
     author = {Joachim A. Hempel},
     title = {A pseudo-trigonometry related to {Ptolemy's} theorem
 and the hyperbolic geometry of punctured spheres},
     journal = {Annales Polonici Mathematici},
     pages = {147--167},
     publisher = {mathdoc},
     volume = {84},
     number = {2},
     year = {2004},
     doi = {10.4064/ap84-2-5},
     language = {en},
     url = {http://geodesic.mathdoc.fr/articles/10.4064/ap84-2-5/}
}