Algorithmic construction of the subdifferential from directional   derivatives
  The subdifferential of a function is a generalization for nonsmooth functions of the concept of gradient. It is frequently used in variational analysis, particularly in the context of nonsmooth optimization. The present work proposes algorithms to reconstruct a polyhedral subdifferential of a function from the computation of finitely many directional derivatives. We provide upper bounds on the required number of directional derivatives when the space is $\R^1$ and $\R^2$, as well as in $\R^n$ where subdifferential is known to possess at most three vertices. 
Charles Audet and Warren Hare
math.OC