On Necessary and Sufficient Conditions for Near-Optimal Singular   Stochastic Controls
  This paper is concerned with necessary and sufficient conditions for near-optimal singular stochastic controls for systems driven by a nonlinear stochastic differential equations (SDEs in short). The proof of our result is based on Ekeland's variational principle and some delicate estimates of the state and adjoint processes. This result is a generalization of Zhou's stochastic maximum principle for near-optimality to singular control problem. 
Mokhtar Hafayed, Syed Abbas, Petr Veverka
math.OC