@article{10_4064_cm_25_2_265_267,
     author = {P. Kostyrko},
     title = {On the symmetric derivative},
     journal = {Colloquium Mathematicum},
     pages = {265--267},
     publisher = {mathdoc},
     volume = {25},
     number = {2},
     year = {1972},
     doi = {10.4064/cm-25-2-265-267},
     language = {en},
     url = {http://geodesic.mathdoc.fr/articles/10.4064/cm-25-2-265-267/}
}