@article{MABA_1990_4_3-4_a10,
     author = {Jan Niechwiej},
     title = {Cosine {Functions} of {Unbounded} {Operators} and {Second} {Order} {Differential} {Equations}},
     journal = {Mathematica balkanica},
     pages = {329},
     publisher = {mathdoc},
     volume = {4},
     number = {3-4},
     year = {1990},
     url = {http://geodesic.mathdoc.fr/item/MABA_1990_4_3-4_a10/}
}