A parabolic action on a proper, CAT(0) cube complex

Abstract: We consider diagram groups as defined by V. Guba and M. Sapir. A diagram group G acts on the associated cube complex K by isometries. It is known that if a cube complex L is of a finite dimension then every isometry g of L is semi-simple, i.e. its translation length is realized. It was conjectured by D. S. Farley that in the case of a diagram group G the action of G on the associated cube complex K is by semisimple isometries even when K has an infinite dimension. In this paper we give a counterexample to Farley Conjecture and we show that R. Thompson's group F, considered as a diagram group, has some elements which act as parabolic (not semi-simple) isometries on the associated cube complex.