Optimal control of linear stochastic evolution equations in Hilbert spaces and uniform observability

In this paper we study the existence of the optimal (minimizing) control for a tracking problem, as well as a quadratic cost problem subject to linear stochastic evolution equations with unbounded coefficients in the drift. The backward differential Riccati equation (BDRE) associated with these problems (see 2], for finite dimensional stochastic equations or 21], for infinite dimensional equations with bounded coefficients) is in general different from the conventional BDRE (see 10], 18]). Under stabilizability and uniform observability conditions and assuming that the control weight-costs are uniformly positive, we establish that BDRE has a unique, uniformly positive, bounded on ℝ ₊ and stabilizing solution. Using this result we find the optimal control and the optimal cost. It is known 18] that uniform observability does not imply detectability and consequently our results are different from those obtained under detectability conditions (see 10]).