@article{EJDE_2001__2001__a99,
     author = {Grammatikopoulos,  Myron K. and Hristov,  Tzvetan D. and Popivanov,  Nedyu I.},
     title = {On the singularities of {3-D} {Protter's} problem for the wave equation},
     journal = {Electronic journal of differential equations},
     year = {2001},
     volume = {2001},
     zbl = {0973.35010},
     language = {en},
     url = {http://geodesic.mathdoc.fr/item/EJDE_2001__2001__a99/}
}