@article{EJDE_2001__2001__a168,
     author = {El Haial,  Abdelillah and Labbas,  Rabah},
     title = {On the ellipticity and solvability of an abstract second-order differential equation},
     journal = {Electronic journal of differential equations},
     year = {2001},
     volume = {2001},
     zbl = {0996.34046},
     language = {en},
     url = {http://geodesic.mathdoc.fr/item/EJDE_2001__2001__a168/}
}