@article{SF_1985__31_134755,
     author = {J.R. Dorroh},
     title = {Semigroups of operators and second-order {Cauchy} problems.},
     journal = {Semigroup forum},
     pages = {297--304},
     year = {1985},
     volume = {31},
     zbl = {0557.47028},
     url = {http://geodesic.mathdoc.fr/item/SF_1985__31_134755/}
}