@article{EJDE_2003__2003__a46,
     author = {Grammatikopoulos,  Myron.K. and Hristov,  Tzvetan D. and Popivanov,  Nedyu I.},
     title = {Singular solutions to {Protter's} problem for the {3-D} wave equation involving lower order terms},
     journal = {Electronic journal of differential equations},
     year = {2003},
     volume = {2003},
     zbl = {1026.35060},
     language = {en},
     url = {http://geodesic.mathdoc.fr/item/EJDE_2003__2003__a46/}
}