differentiability; convex functions
We characterize sets of non-differentiability points of convex functions on $\Bbb R^n$. This completes (in $\Bbb R^n$) the result by Zajíček  which gives a characterization of the magnitude of these sets.
 Zajíček L.: On the differentiability of convex functions in finite and infinite dimensional spaces
. Czechoslovak Math. J. 29 (104) (1979), 340-348. MR 0536060