@article{10_4064_cm_10_2_281_285,
     author = {K. Krzy\.zewski},
     title = {Note on approximative derivative},
     journal = {Colloquium Mathematicum},
     pages = {281--285},
     publisher = {mathdoc},
     volume = {10},
     number = {2},
     year = {1963},
     doi = {10.4064/cm-10-2-281-285},
     language = {en},
     url = {http://geodesic.mathdoc.fr/articles/10.4064/cm-10-2-281-285/}
}