Geodesic
The Symetrie Ι-Approximate Derivatives
Acta Universitatis Lodziensis. Folia Mathematica, Tome 03 (1989) 
Libicka, Inga; Łazarow, Ewa. The Symetrie Ι-Approximate Derivatives. Acta Universitatis Lodziensis. Folia Mathematica, Tome 03 (1989). http://geodesic.mathdoc.fr/item/AULFM_1989_03_a5/
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     author = {Libicka, Inga and {\L}azarow, Ewa},
     title = {The {Symetrie} {I-Approximate} {Derivatives}},
     journal = {Acta Universitatis Lodziensis. Folia Mathematica},
     year = {1989},
     volume = {03},
     language = {pl},
     url = {http://geodesic.mathdoc.fr/item/AULFM_1989_03_a5/}
}
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Voir la notice de l'article provenant de la source University of Lodz Repository

In this paper we shall give a definition of a symmetric Ι-approximate derivative of a function f: R → R. We shall prove several properties of its Ι-approximate derivative.