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@article{MS2_1962__11_165821, author = {Erik Balslev}, title = {Perturbation of {Ordinary} {Differential} {Operators.}}, journal = {Mathematica Scandinavica}, pages = {131--148}, publisher = {mathdoc}, volume = {11}, year = {1962}, zbl = {0196.44501}, url = {http://geodesic.mathdoc.fr/item/MS2_1962__11_165821/} }
Erik Balslev. Perturbation of Ordinary Differential Operators.. Mathematica Scandinavica, Tome 11 (1962), pp. 131-148. http://geodesic.mathdoc.fr/item/MS2_1962__11_165821/