@article{TMF_2016_186_1_a5,
     author = {A. K. Motovilov},
     title = {Alternative proof of the~a~priori $\tan\Theta$ theorem},
     journal = {Teoreti\v{c}eska\^a i matemati\v{c}eska\^a fizika},
     pages = {101--112},
     publisher = {mathdoc},
     volume = {186},
     number = {1},
     year = {2016},
     language = {ru},
     url = {http://geodesic.mathdoc.fr/item/TMF_2016_186_1_a5/}
}