Unitarity and irreversibility in chaotic systems.

We analyze the spectral properties of the Perron-Frobenius operator, U, associated with some simple highly chaotic maps. We obtain a spectral decomposition of U in terms of generalized eigenfunctions of U and its adjoint. The corresponding eigenvalues are related to the decay rates of correlation functions and have magnitude less than one, so that physically measurable quantities manifestly approach equilibrium. To obtain decaying eigenstates of unitary and isometric operators it is necessary to extend the Hilbert space formulation of dynamical systems. We describe and illustrate a method to obtain the decomposition explicitly.