TILINGS, FUNDAMENTAL COCYCLES AND FUNDAMENTAL GROUPS OF SYMBOLIC Zd-ACTIONS

We prove that certain topologically mixing two-dimensional shifts of finite type have a ‘fundamental’ 1-cocycle with the property that every continuous 1-cocycle on the shift space with values in a discrete group is continuously cohomologous to a homomorphic image of the fundamental cocycle. These fundamental cocycles are closely connected with representations of the shift space by Wang tilings and the tiling groups of J.H. Conway, J.C. Lagarias and W. Thurston, and they determine the projective fundamental groups of the shift spaces introduced by W. Geller and J. Propp.