@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},
     publisher = {mathdoc},
     volume = {2001},
     year = {2001},
     language = {en},
     url = {http://geodesic.mathdoc.fr/item/EJDE_2001__2001__a99/}
}