Jouak, Mohamed, and Lionel Thibault. “Directional Derivates and Almost Everywhere Differentiability of Biconvex and Concave-Convex Operators”. Mathematica Scandinavica 57 (December 1, 1985): 215–224. Accessed September 8, 2026. https://journals.msp.org/mscand/article/view/1555.