@article{MZ_1957__67_169671,
     author = {G.S.S. Ludford and S.H. Schot},
     title = {On {Sonic} {Limit} {Lines} in the {Hodograph} {Method.}},
     journal = {Mathematische Zeitschrift},
     pages = {229--237},
     publisher = {mathdoc},
     volume = {67},
     year = {1957},
     zbl = {0083.41103},
     url = {http://geodesic.mathdoc.fr/item/MZ_1957__67_169671/}
}