@article{SM_1983_46_3_a4,
     author = {S. I. Piskarev},
     title = {Periodic and almost periodic cosine operator functions},
     journal = {Sbornik. Mathematics},
     pages = {391--402},
     publisher = {mathdoc},
     volume = {46},
     number = {3},
     year = {1983},
     language = {en},
     url = {http://geodesic.mathdoc.fr/item/SM_1983_46_3_a4/}
}