@article{10_4064_cm_54_1_139_142,
     author = {H. H. Pu and H. W. Pu},
     title = {On level sets of approximate derivative and approximate symmetric derivative},
     journal = {Colloquium Mathematicum},
     pages = {139--142},
     publisher = {mathdoc},
     volume = {54},
     number = {1},
     year = {1987},
     doi = {10.4064/cm-54-1-139-142},
     language = {en},
     url = {http://geodesic.mathdoc.fr/articles/10.4064/cm-54-1-139-142/}
}