Random M\"obius dynamics on the unit disc and perturbation theory for Lyapunov exponents

Randomly drawn $2\times 2$ matrices induce a random dynamics on the Riemann sphere via the M"obius transformation. Considering a situation where this dynamics is restricted to the unit disc and given by a random rotation perturbed by further random terms depending on two competing small parameters, the invariant (Furstenberg) measure of the random dynamical system is determined. The results have applications to the perturbation theory of Lyapunov exponents which are of relevance for one-dimensional discrete random Schr"odinger operators.