@article{IM2_1968_2_1_a7,
     author = {A. V. Strauss},
     title = {On the extensions and the characteristic function of a~symmetric operator},
     journal = {Izvestiya. Mathematics },
     pages = {181--203},
     publisher = {mathdoc},
     volume = {2},
     number = {1},
     year = {1968},
     language = {en},
     url = {http://geodesic.mathdoc.fr/item/IM2_1968_2_1_a7/}
}