Biharmonic functions on groups and limit theorems for quasimorphisms along random walks

Abstract

We show for very general classes of measures on locally compact second countable groups that every Borel measurable quasimorphism is at bounded distance from a quasi-biharmonic one. This allows us to deduce nondegenerate central limit theorems and laws of the iterated logarithm for such quasimorphisms along regular random walks on topological groups using classical martingale limit theorems of Billingsley and Stout. For quasi-biharmonic quasimorphisms on countable groups we also obtain integral representations using martingale convergence.