Cohomological Reduction for Fiber-Contracting Extensions:From Subcohomology to Thermodynamic Formalism

Abstract

We develop a reduction and transfer framework for cohomological, variational, and thermodynamic problems in fiber-contracting extensions of local homeomorphisms. Under uniform contraction along the fibers and the existence of a continuous global section, every H\"older potential admits the explicit decomposition \[ \varphi=\psi\circ\pi+u-u\circ F, \] where $\psi$ is defined on the quotient and $u$ is obtained by a uniformly convergent fiberwise series. We give quantitative criteria for continuity and H\"older regularity and use the reduction to transfer subcohomological and Liv\v{s}ic-type statements from the quotient to the extension. We prove that invariant measures on the extension and quotient are in bijection, invariant averages are preserved, and the fibers have zero relative topological entropy. Consequently, metric entropy, ergodic optimization, pressure, minimizing measures, and equilibrium states correspond. We further establish weak Gibbs transfer under subexponential comparison of Bowen-ball masses and prove blockwise Bowen sequential weak Gibbs transfer along prescribed point-dependent Gibbs times; under uniformly bounded sequential mass distortion, the strong Bowen sequential Gibbs property is preserved. Concrete criteria for Bowen-ball comparability are provided. Finally, affine and symbolic contracting skew-products illustrate the framework, including the explicit reduction \[ \varphi(x,y)=g(x)+by \quad\;\;\;\;\;\;\;\;\quad \psi(x)=g(x)+\frac{b}{1-a}\rho(x) \] for $F(x,y)=(f(x),ay+\rho(x))$, $|a|<1$.