Spectral properties of the Ruelle operator for product-type potentials on shift spaces:

We study a class of potentials f on one-sided full shift spaces over finite or countable alphabets, called potentials of product type. We obtain explicit formulae for the leading eigenvalue, the eigenfunction (which may be discontinuous) and the eigenmeasure of the Ruelle operator. The uniqueness property of these quantities is also discussed, and it is shown that there always exists a Bernoulli equilibrium state even if f does not satisfy Bowen's condition. We apply these results to potentials f:{-1,1}N→R of the form f(x1,x2,⋯)=x1+2-γx2+3-γx3++n-γxn+with γ>1. For 3/2<γ≤2, we obtain the existence of two different eigenfunctions. Both functions are (locally) unbounded and exist almost surely (but not everywhere) with respect to the eigenmeasure and the measure of maximal entropy, respectively.