@article{TMF_1988_76_1_a11,
     author = {V. V. Val'kov},
     title = {Unitary transformations of the group~$U(N)$ and diagonalization of multilevel {Hamiltonians}},
     journal = {Teoreti\v{c}eska\^a i matemati\v{c}eska\^a fizika},
     pages = {143--152},
     publisher = {mathdoc},
     volume = {76},
     number = {1},
     year = {1988},
     language = {ru},
     url = {http://geodesic.mathdoc.fr/item/TMF_1988_76_1_a11/}
}