@incollection{MEM_2012_41_1_a18,
     author = {Popivanov, Nedyu and Nikolov, Aleksey},
     title = {Singular {Solutions} of {Protter{\textquoteright}s} {Problem} for a {Class} of {3-D} {Hyperbolic} {Equations}},
     booktitle = {},
     series = {Mathematics and Education in Mathematics},
     pages = {191--196},
     year = {2012},
     volume = {41},
     number = {1},
     language = {en},
     url = {http://geodesic.mathdoc.fr/item/MEM_2012_41_1_a18/}
}