Nonhyperbolic Step Skew-products: Entropy Spectrum of Lyapunov Exponents

We study the fiber Lyapunov exponents of step skew-product maps over a complete shift of N , N ≥ 2, symbols and with C1 diffeomorphisms of the circle as fiber maps. The systems we study are transitive and genuinely nonhyperbolic, exhibiting simultaneously ergodic measures with positive, negative, and zero exponents. We derive a multifractal analysis for the topological entropy of the level sets of Lyapunov exponent. The results are formulated in terms of Legendre-Fenchel transforms of restricted variational pressures, considering hyperbolic ergodic measures only, as well as in terms of restricted variational principles of entropies of ergodic measures with given exponent. We show that the entropy of the level sets is a continuous function of the Lyapunov exponent. The level set of zero exponent has positive, but not maximal, topological entropy. Under the additional assumption of proximality, there exist two unique ergodic measures of maximal entropy, one with negative and one with positive fiber Lyapunov exponent.