@article{IM_1990__101_2_143795,
     author = {S. Popa},
     title = {Classification of subfactors: the reduction to commuting squares.},
     journal = {Inventiones mathematicae},
     pages = {19--44},
     publisher = {mathdoc},
     volume = {101},
     number = {2},
     year = {1990},
     zbl = {0757.46054},
     url = {http://geodesic.mathdoc.fr/item/IM_1990__101_2_143795/}
}