@article{10_4064_ap92_1_5,
     author = {A. G. Ramm},
     title = {A symmetry problem},
     journal = {Annales Polonici Mathematici},
     pages = {49--54},
     publisher = {mathdoc},
     volume = {92},
     number = {1},
     year = {2007},
     doi = {10.4064/ap92-1-5},
     language = {en},
     url = {http://geodesic.mathdoc.fr/articles/10.4064/ap92-1-5/}
}