On some properties of the Lyapunov equation for damped systems

We consider a damped linear vibrational system whose dampers depend linearly on the viscosity parameter v. We show that the trace of the corresponding Lyapunov solution can be represented as a rational function of v whose poles are the eigenvalues of a certain skew symmetric matrix. This makes it possible to derive an asymptotic expansion of the solution in the neighborhood of zero (small damping).