@article{MZ_1981__178_173114,
     author = {Lutz Weis},
     title = {Perturbation {Classes} of {Semi-Fredholm} {Operators.}},
     journal = {Mathematische Zeitschrift},
     pages = {429--442},
     publisher = {mathdoc},
     volume = {178},
     year = {1981},
     zbl = {0541.47010},
     url = {http://geodesic.mathdoc.fr/item/MZ_1981__178_173114/}
}