A General Maximum Principle for Stochastic Systems with Delay

In this paper, we consider optimal control problems derived by stochastic systems with delay, where control domains are non-convex and the diffusion coefficients depend on control variables. By an estimate of the integral of x_{1}(t)x_{1}(t-\delta) term, we obtain a general maximum principle for the optimal control problems with a standard spike variational technique and duality method. The maximum principle is applied to study a delayed linear-quadratic optimal control problem with a non-convex control domain; an optimal solution is obtained.