. We introduce and study finite subdivision rules. A finite subdivision rule is a finite list of instructions which determines a subdivision of a given planar tiling. Given a finite subdivision rule and a planar tiling associated to it, we obtain an infinite sequence of tilings by recursively subdividing the given tiling. We wish to determine when this sequence of tilings is conformal in the sense of Cannon's combinatorial Riemann mapping theorem. In this setting, it is proved that the two axioms of conformality can be repaced by a single axiom which is implied by either of them, and that it su#ces to check conformality for finitely many test annuli. Theorems are given which show how to exploit symmetry, and many examples are computed. This paper is concerned with recursive subdivisions of planar complexes. As an introductory example, we present a finite subdivision rule in Figure 1. There are two kinds of edges and three kinds of tiles. A thin edge is subdivided into five su...

3. Why Call it Hyperbolic Geometry? 63 4. Understanding the One-Dimensional Case 65

Abstract. Suppose R is an orientation-preserving finite subdivision rule with an edge pairing. Then the subdivision map σR is either a homeomorphism, a covering of a torus, or a critically finite branched covering of a 2-sphere. If R has mesh approaching 0 and SR is a 2-sphere, it is proved in Theorem 3.1 that if R is conformal then σR is realizable by a rational map. Furthermore, a general construction is given which, starting with a one tile rotationally invariant finite subdivision rule, produces a finite subdivision rule Q with an edge pairing such that σQ is realizable by a rational map. In this paper we illustrate a technique for constructing critically finite rational maps. The starting point for the construction is an orientation-preserving finite subdivision rule R with an edge pairing. For such a finite subdivision rule the CW complex SR is a surface, and the map σR: SR → SR is a branched covering. If SR is orientable, then unless σR is a homeomorphism or a covering of the torus, SR is a 2-sphere and σR is critically finite. In the latter case, SR has an orbifold structure OR and σR induces a map τR: T (OR) →T(OR) on the Teichmüller space of the orbifold. By work of Thurston, σR can be realized by a rational map exactly if τR has a fixed point. Alternatively, we prove in Theorem 3.1 that σR can be realized by a rational map if R has mesh approaching 0 and is conformal. We next give a general construction which, starting with a one tile rotationally invariant finite subdivision rule R, produces an orientation-preserving finite subdivision rule Q with an edge pairing such that Q is conformal if and only if R is conformal; we then show in Theorem 3.2 that σQ is realizable by a rational map. We next give several examples of orientation-preserving finite subdivision rules with edge pairings. For each example R for which the associated map σR can be realized by a rational map, we explicitly construct a rational map realizing it. We conclude with some questions. A motivation for this work is the Bowers-Stephenson paper [1]. In that paper they construct an expansion complex for the pentagonal subdivision rule (see Figure 4) and numerically approximate the expansion constant. In Example 4.4 we consider an associated finite subdivision rule Q with an edge pairing and construct a rational map fQ(z) =

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Abstract. This paper develops the basic theory of conformal structures on finite subdivision rules. The work depends heavily on the use of expansion complexes, which are defined and discussed in detail. It is proved that a finite subdivision rule with bounded valence and mesh approaching 0 is conformal (in the combinatorial sense) if there is a conformal structure on the model subdivision complex with respect to which the subdivision map is conformal. This gives a new approach to the difficult combinatorial problem of determining when a finite subdivision rule is conformal. 1.

Abstract. This paper gives applications of earlier work of the authors on the use of expansion complexes for studying conformality of finite subdivision rules. The first application is that a one-tile rotationally invariant finite subdivision rule (with bounded valence and mesh approaching 0) has an invariant conformal structure, and hence is conformal. The paper next considers one-tile single valence finite subdivision rules. It is shown that an expansion map for such a finite subdivision rule can be conjugated to a linear map, and that the finite subdivision rule is conformal exactly when this linear map is either a dilation or has eigenvalues that are not real. Finally, an example is given of an irreducible finite subdivision rule that has a parabolic expansion complex and a hyperbolic expansion complex. 1.