Virtual homological eigenvalues and the Weil-Petersson translation length

Abstract

For any pseudo-Anosov automorphism on an orientable closed surface, an inequality is established by bounding certain growth of virtual homological eigenvalues with the Weil-Petersson translation length. The new inequality fits nicely with other known inequalities due to Kojima and McShane (2018) and Lê (2014). The new quantity to be considered is the square sum of the logarithmic radii of the homological eigenvalues (with multiplicity) outside the complex unit circle, called the homological Jensen square sum. The main theorem is as follows. For any cofinal sequence of regular finite covers of a given surface, together with lifts of a given pseudo-Anosov, the homological Jensen square sum of the lifts grows at most linearly fast compared with the covering degree, and the square root of the growth rate is at most $1/\sqrt {4\pi }$ times the Weil-Petersson translation length of the given pseudo-Anosov.