Gap Labelling for Schr\"odinger Operators on Quasiperiodic Tilings
  For a large class of tilings, including those which are obtained by the generalized dual method from regular grids, it is shown that their algebra is stably isomorphic to a crossed product with $\Z^d$. Penrose tilings belong to this class. This enlarges the class of tilings of which can be shown that a set of possible gap labels is completely determined by an invariant measure on the hull. 
Johannes Kellendonk