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JAMES J WALTONPattern-equivariant (PE) cohomology is a well-established tool with which to interpret theČech cohomology groups of a tiling space in a highly geometric way. We consider homology groups of PE infinite chains and establish Poincaré duality between the PE cohomology and PE homology. The Penrose kite and dart tilings are taken as our central running example; we show how through this formalism one may give highly approachable geometric descriptions of the generators of theČech cohomology of their tiling space. These invariants are also considered in the context of rotational symmetry. Poincaré duality fails over integer coefficients for the "ePE homology groups" based upon chains which are PE with respect to orientation-preserving Euclidean motions between patches. As a result we construct a new invariant, which is of relevance to the cohomology of rotational tiling spaces. We present an efficient method of computation of the PE and ePE (co)homology groups for hierarchical tilings.52C23; 37B50, 52C22, 55N05
IntroductionIn the past few decades a rich class of highly ordered patterns has emerged whose central examples, despite lacking global translational symmetries, exhibit intricate internal structure, imbuing these patterns with properties akin to those enjoyed by periodically repeating patterns. The field of aperiodic order aims to study such patterns, and to establish connections between their properties, and their constructions, to other fields of mathematics and the natural sciences. PE cohomology allows for an intuitive description of theČech cohomology L H . / of tiling spaces. Over R coefficients the PE cochain groups may be defined using PE differential forms [24], and over general abelian coefficients, when the tiling has a cellular structure, with PE cellular cochains; see Sadun [37]. Rather than just providing a reflection of topological invariants of tiling spaces, on the contrary, these PE invariants are of principal relevance to aperiodic structures and their connections with other fields in their own right; see, for example, Kelly and Sadun's use of them [29] in a topological proof of theorems of Kesten and Oren regarding the discrepancy of irrational rotations. It is perhaps more appropriate to view the isomorphism between L H . / and the PE cohomology as an elegant interpretation of the PE cohomology, rather than vice versa, of theoretical and computational importance.In this paper we introduce the pattern-equivariant homology gro...