@article{SM_1971_13_4_a7,
     author = {I. Ts. Gokhberg and E. I. Sigal},
     title = {An operator generalization of the logarithmic residue theorem and the theorem of {Rouch\'e}},
     journal = {Sbornik. Mathematics},
     pages = {603--625},
     publisher = {mathdoc},
     volume = {13},
     number = {4},
     year = {1971},
     language = {en},
     url = {http://geodesic.mathdoc.fr/item/SM_1971_13_4_a7/}
}