Smith method for projected Lyapunov and Stein equations

We consider the projected Lyapunov and Stein equations arising in model order reduction and optimal control of descriptor systems. The projected Lyapunov equation is transformed to an equivalent projected Stein equation then solved by a generalized Smith iterative method. For a projected general Stein equation with a singular matrix "E", a double Cayley transform is devised to remove the singularity, and then the generalized Smith method is applied. Numerical examples are provided to demonstrate the feasibility and efficiency of our approach.