@article{JDM_1909__18_145180,
     author = {Julius Perl},
     title = {Bemerkungen zur {Formel} (...)2 + (...)2 = 1.},
     journal = {Jahresbericht der Deutschen Mathematiker-Vereinigung},
     pages = {399--401},
     year = {1909},
     volume = {18},
     url = {http://geodesic.mathdoc.fr/item/JDM_1909__18_145180/}
}