@article{EXMATH_1989__4_3_38327,
     author = {Jos\'e A. Alvarez and Teresa Alvarez and Manuel Gonz\'alez},
     title = {Gap and perturbation of {non-semi-Fredholm} operators.},
     journal = {Extracta Mathematicae},
     pages = {139},
     publisher = {mathdoc},
     volume = {4},
     number = {3},
     year = {1989},
     language = {en},
     url = {https://geodesic-test.mathdoc.fr/item/EXMATH_1989__4_3_38327/}
}