Charles Radin, Lorenzo Sadun
AN ALGEBRAIC INVARIANT FOR SUBSTITUTION TILING SYSTEMS
(1021K, postscript)

ABSTRACT.  We consider tilings of Euclidean spaces by polygons or polyhedra, in
particular, tilings made by a substitution process, such as the
Penrose tilings of the plane. We define an isomorphism invariant
related to a subgroup of rotations and compute it for various
examples. We also extend our analysis to more general dynamical
systems.