  We generalize Wonham's theorem on solvability of algebraic operator Riccati
equations to Banach spaces, namely there is a unique stabilizing solution to
A*P+PA-PBB*P+C*C=0 when (A,B) is exponentially stabilizable and (C,A) is
exponentially detectable. The proof is based on a new approach that treats the
linear part of the equation as the generator of a positive semigroup on the
space of symmetric operators from a Banach space to its dual, and the quadratic
part as an order concave map. A direct analog of global Newton's iteration for
concave functions is then used to approximate the solution, the approximations
converge in the strong operator topology, and the convergence is monotone. The
linearized equations are the well-known Lyapunov equations of the form
A*P+PA=-Q, and semigroup stability criterion in terms of them is also
generalized.
