@article{EJDE_2007__2007__a216,
     author = {Tarama,  Shigeo},
     title = {Energy estimate for wave equations with coefficients in some {Besov} type class},
     journal = {Electronic journal of differential equations},
     year = {2007},
     volume = {2007},
     zbl = {1137.35319},
     language = {en},
     url = {http://geodesic.mathdoc.fr/item/EJDE_2007__2007__a216/}
}