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@article{PORMA_1956__15_1_114764, author = {Austin, D.G.}, title = {A {Lipschitzian} characterization of approximately differentiable functions}, journal = {Portugaliae mathematica}, pages = {19--29}, publisher = {mathdoc}, volume = {15}, number = {1}, year = {1956}, mrnumber = {0084024}, zbl = {0072.27904}, language = {en}, url = {http://geodesic.mathdoc.fr/item/PORMA_1956__15_1_114764/} }
Austin, D.G. A Lipschitzian characterization of approximately differentiable functions. Portugaliae mathematica, Tome 15 (1956) no. 1, pp. 19-29. http://geodesic.mathdoc.fr/item/PORMA_1956__15_1_114764/