Aperiodic tilings (tutorial) Boris Solomyak U Washington and Bar-Ilan February 12, 2015, ICERM Boris Solomyak (U Washington and Bar-Ilan) Aperiodic tilings February 12, 2015, ICERM 1 / 45
Plan of the talk 1 Introduction. 2 Tiling definitions, tiling spaces, tiling dynamical systems. 3 Spectral theory. Boris Solomyak (U Washington and Bar-Ilan) Aperiodic tilings February 12, 2015, ICERM 2 / 45
I. Aperiodic tilings: some references C. Radin, Miles of Tiles , AMS Student Math. Library, vol. I, 1999. L. Sadun, Topology of Tiling Spaces , AMS University Lecture Series, vol. 46, 2008. M. Baake and U. Grimm, Aperiodic Order. Vol. 1: A Mathematical Invitation , Cambridge, 2013. E. Harriss and D. Frettl¨ oh, Tilings Encyclopedia , http://tilings.math.uni-bielefeld.de Boris Solomyak (U Washington and Bar-Ilan) Aperiodic tilings February 12, 2015, ICERM 3 / 45
I. Aperiodic tilings A tiling (or tesselation) of R d is a collection of sets, called tiles, which have nonempty disjoint interiors and whose union is the entire R d . Aperiodic set of tiles can tile the space, but only non-periodically. Boris Solomyak (U Washington and Bar-Ilan) Aperiodic tilings February 12, 2015, ICERM 4 / 45
I. Aperiodic tilings A tiling (or tesselation) of R d is a collection of sets, called tiles, which have nonempty disjoint interiors and whose union is the entire R d . Aperiodic set of tiles can tile the space, but only non-periodically. Origins in Logic: Hao Wang (1960’s) asked if it is decidable whether a given set of tiles (square tiles with marked edges) can tile the plane? R. Berger (1966) proved undecidability, and in the process constructed an aperiodic set of 20,426 Wang tiles. R. Robinson (1971) found an aperiodic set of 6 tiles (up to isometries). Boris Solomyak (U Washington and Bar-Ilan) Aperiodic tilings February 12, 2015, ICERM 4 / 45
I. Penrose tilings One of the most interesting aperiodic sets is the set of Penrose tiles , discovered by Roger Penrose (1974) . Penrose tilings play a central role in the theory because they can be generated by any of the three main methods: 1 local matching rules (“jigsaw puzzle”); 2 tiling substitutions; 3 projection method (projecting a “slab” of a periodic structure in a higher-dimensional space to the plane). Boris Solomyak (U Washington and Bar-Ilan) Aperiodic tilings February 12, 2015, ICERM 5 / 45
I. Penrose and his tiles Figure: Sir Roger Penrose Figure: Penrose rhombi Boris Solomyak (U Washington and Bar-Ilan) Aperiodic tilings February 12, 2015, ICERM 6 / 45
I. Penrose tiling Figure: A patch of the Penrose tiling Boris Solomyak (U Washington and Bar-Ilan) Aperiodic tilings February 12, 2015, ICERM 7 / 45
I. Penrose tiling (kites and darts) Boris Solomyak (U Washington and Bar-Ilan) Aperiodic tilings February 12, 2015, ICERM 8 / 45
I. Penrose tiling: basic properties Non-periodic: no translational symmetries. Hierarchical structure, “self-similarity,” or “composition”; can be obtained by a simple “inflate-and-subdivide” process. This is how one can show that the tiling of the entire plane exists. “Repetitivity” and uniform pattern frequency: every pattern that appears somewhere in the tiling appears throughout the plane, in a relatively dense set of locations, even with uniform frequency. 5-fold (even 10-fold) rotational symmetry: every pattern that appears somewhere in the tiling also appears rotated by 36 degrees, and with the same frequency (impossible for a periodic tiling). Boris Solomyak (U Washington and Bar-Ilan) Aperiodic tilings February 12, 2015, ICERM 9 / 45
I. Quasicrystals Figure: Dani Schechtman (2011 Chemistry Nobel Prize); quasicrystal diffraction pattern (below) Figure: quasicrystal diffraction pattern Boris Solomyak (U Washington and Bar-Ilan) Aperiodic tilings February 12, 2015, ICERM 10 / 45
I. Quasicrystals (aperiodic crystals) Quasicrystals were discovered by D. Schechtman (1982) . A quasicrystal is a solid (usually, metallic alloy) which, like a crystal, has a sharp X-ray diffraction pattern, but unlike a crystal, has an aperiodic atomic structure. Aperiodicity was inferred from a “forbidden” 10-fold symmetry of the diffraction picture. Boris Solomyak (U Washington and Bar-Ilan) Aperiodic tilings February 12, 2015, ICERM 11 / 45
I. Quasicrystals (aperiodic crystals) Quasicrystals were discovered by D. Schechtman (1982) . A quasicrystal is a solid (usually, metallic alloy) which, like a crystal, has a sharp X-ray diffraction pattern, but unlike a crystal, has an aperiodic atomic structure. Aperiodicity was inferred from a “forbidden” 10-fold symmetry of the diffraction picture. Other types of quasicrystals have been discovered by T. Ishimasa, H. U. Nissen, Y. Fukano (1985) and others (Al-Mn alloy, 10-fold symmetry, Ni-Cr alloy, 12-fold symmetry; V-Ni-Si alloy, 8-fold symmetry) Boris Solomyak (U Washington and Bar-Ilan) Aperiodic tilings February 12, 2015, ICERM 11 / 45
I. Other quasicrystal diffraction patterns Figure: From the web site of Uwe Grimm (http://mcs.open.ac.uk/ugg2/quasi.shtml) Boris Solomyak (U Washington and Bar-Ilan) Aperiodic tilings February 12, 2015, ICERM 12 / 45
I. Substitutions Symbolic substitutions have been studied in Dynamics (coding of geodesics), Number Theory, Automata Theory, and Combinatorics of Words for a long time. Boris Solomyak (U Washington and Bar-Ilan) Aperiodic tilings February 12, 2015, ICERM 13 / 45
I. Substitutions Symbolic substitutions have been studied in Dynamics (coding of geodesics), Number Theory, Automata Theory, and Combinatorics of Words for a long time. Symbolic substitution is a map ζ from a finite “alphabet” { 0 , . . . , m − 1 } into the set of “words” in this alphabet. Thue-Morse: ζ (0) = 01 , ζ (1) = 10. Iterate (by concatenation) : 0 → 01 → 0110 → 01101001 → . . . n →∞ ζ n (0) ∈ { 0 , 1 } N , u = ζ ( u ) u = u 0 u 1 u 2 . . . = lim Fibonacci: ζ (0) = 01 , ζ (1) = 0. Iterate (by concatenation) : 0 → 01 → 010 → 01001 → . . . u = 0100101001001010010100100 . . . Boris Solomyak (U Washington and Bar-Ilan) Aperiodic tilings February 12, 2015, ICERM 13 / 45
I. Tile-substitutions in R d Symbolic substitutions have been generalized to higher dimensions. One can just consider higher-dimensional symbolic arrays, e.g. 0 0 1 1 0 → 1 → 1 , 0 1 0 Boris Solomyak (U Washington and Bar-Ilan) Aperiodic tilings February 12, 2015, ICERM 14 / 45
I. Tile-substitutions in R d Symbolic substitutions have been generalized to higher dimensions. One can just consider higher-dimensional symbolic arrays, e.g. 0 0 1 1 0 → 1 → 1 , 0 1 0 More interestingly, one can consider “geometric” substitutions, with the symbols replaced by tiles. Penrose tilings can be obtained in such a way. Boris Solomyak (U Washington and Bar-Ilan) Aperiodic tilings February 12, 2015, ICERM 14 / 45
I. Example: chair tiling Examples are taken from ”Tiling Encyclopedia”, see http://tilings.math.uni-bielefeld.de/ Figure: tile-substitution, real expansion constant λ = 2 Boris Solomyak (U Washington and Bar-Ilan) Aperiodic tilings February 12, 2015, ICERM 15 / 45
I. Example: chair tiling Boris Solomyak (U Washington and Bar-Ilan) Aperiodic tilings February 12, 2015, ICERM 16 / 45
I. Example: Ammann-Beenker rhomb-triangle tiling √ Figure: tile-substitution, real expansion constant λ = 1 + 2 Boris Solomyak (U Washington and Bar-Ilan) Aperiodic tilings February 12, 2015, ICERM 17 / 45
I. Example: Ammann-Beenker rhomb-triangle tiling Boris Solomyak (U Washington and Bar-Ilan) Aperiodic tilings February 12, 2015, ICERM 18 / 45 Figure: patch of the tiling
I. Substitution tilings in R d One can even consider tilings with fractal boundary. Boris Solomyak (U Washington and Bar-Ilan) Aperiodic tilings February 12, 2015, ICERM 19 / 45
I. Substitution tilings in R d One can even consider tilings with fractal boundary. In fact, such tilings arise naturally in connection with Markov partitions for hyperbolic toral automorphisms in dimensions 3 and larger Numeration systems with a complex base. Boris Solomyak (U Washington and Bar-Ilan) Aperiodic tilings February 12, 2015, ICERM 19 / 45
I. Substitution tilings in R d One can even consider tilings with fractal boundary. In fact, such tilings arise naturally in connection with Markov partitions for hyperbolic toral automorphisms in dimensions 3 and larger Numeration systems with a complex base. Rauzy tiling is a famous example. let λ be the complex root of 1 − z − z 2 − z 3 = 0 with positive imaginary part, z ≈ − 0 . 771845 + 1 . 11514 i . Then (Lebesgue) almost every ζ ∈ C has a unique representation ∞ � a n λ − n , ζ = n = − N where a n ∈ { 0 , 1 } , a n a n +1 a n +2 = 0 for all n . Boris Solomyak (U Washington and Bar-Ilan) Aperiodic tilings February 12, 2015, ICERM 19 / 45
Rauzy tiles Boris Solomyak (U Washington and Bar-Ilan) Aperiodic tilings February 12, 2015, ICERM 20 / 45
Gerard Rauzy Figure: Gerard Rauzy (1938-2010) Boris Solomyak (U Washington and Bar-Ilan) Aperiodic tilings February 12, 2015, ICERM 21 / 45
II. Tiling definitions Prototile set: A = { A 1 , . . . , A N } , compact sets in R d , which are closures of its interior; interior is connected. (May have “colors” or “labels”.) Boris Solomyak (U Washington and Bar-Ilan) Aperiodic tilings February 12, 2015, ICERM 22 / 45
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