@article{MS2_1985__57_166950,
     author = {Mohamed Jouak and Lionel Thibault},
     title = {Directional derivates and almost everywhere differentiability of biconvex and concave-convex operators.},
     journal = {Mathematica Scandinavica},
     pages = {215--224},
     publisher = {mathdoc},
     volume = {57},
     year = {1985},
     zbl = {0608.46022},
     url = {http://geodesic.mathdoc.fr/item/MS2_1985__57_166950/}
}