@article{JCA_1997_4_2_a0,
     author = {F. H. Clarke and Yu. S. Ledyaev and R. J. Stern},
     title = {Complements, {Approximations,} {Smoothings} and {Invariance} {Properties}},
     journal = {Journal of convex analysis},
     pages = {189--22},
     publisher = {mathdoc},
     volume = {4},
     number = {2},
     year = {1997},
     url = {http://geodesic.mathdoc.fr/item/JCA_1997_4_2_a0/}
}