@article{10_4153_CJM_1980_020_7,
     author = {Rockafellar, R. T.},
     title = {Generalized {Directional} {Derivatives} and {Subgradients} of {Nonconvex} {Functions}},
     journal = {Canadian journal of mathematics},
     pages = {257--280},
     year = {1980},
     volume = {32},
     number = {2},
     doi = {10.4153/CJM-1980-020-7},
     url = {http://geodesic.mathdoc.fr/articles/10.4153/CJM-1980-020-7/}
}