Circle homeomorphisms with square summable diamond shears

Abstract

We introduce and the study the space of homeomorphisms of the circle (up to M\"obius transformations) which are in $\ell^2$ with respect to modular coordinates called diamond shears along the edges of the Farey tessellation. Diamond shears are related combinatorially to shear coordinates, and are also closely related to the $\log \Lambda$-lengths of decorated Teichm\"uller space introduced by Penner. We obtain sharp results comparing this new class to the Weil-Petersson class and H\"older classes of circle homeomorphisms. We also express the Weil-Petersson metric tensor and symplectic form in terms of infinitesimal shears and diamond shears.