@article{IM2_1977_11_4_a7,
     author = {A. L. Likhtarnikov and V. A. Yakubovich},
     title = {The frequency theorem for continuous one-parameter semigroups},
     journal = {Izvestiya. Mathematics},
     pages = {849--864},
     year = {1977},
     volume = {11},
     number = {4},
     language = {en},
     url = {http://geodesic.mathdoc.fr/item/IM2_1977_11_4_a7/}
}