Closed-form expressions for projectors onto polyhedral sets in Hilbert   spaces
  We provide formulas for projectors onto a polyhedral set, i.e. the intersection of a finite number of halfspaces. To this aim we formulate the problem of finding the projection as a convex optimization problem and we solve explicitly sufficient and necessary optimality conditions. This approach has already been successfully applied in deriving formulas for projection onto the intersection of two halfspaces. We also discuss possible generalizations to Banach spaces. 
Krzysztof E. Rutkowski
math.OC