@article{JCA_2011_18_2_JCA_2011_18_2_a0,
     author = {S. Dolecki and G. H. Greco},
     title = {Tangency vis-\`a-vis {Differentiability} by {Peano,} {Severi} and {Guareschi}},
     journal = {Journal of convex analysis},
     pages = {301--339},
     publisher = {mathdoc},
     volume = {18},
     number = {2},
     year = {2011},
     url = {http://geodesic.mathdoc.fr/item/JCA_2011_18_2_JCA_2011_18_2_a0/}
}