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# order beyond periodicity

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order beyond periodicity

### order beyond periodicity

Christoph Richard, FAU Erlangen-N¨urnberg Seminaire Lotharingien de Combinatoire, March 2015

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order beyond periodicity motivation

### motivation

every lattice Λ⊂Rd is

uniformly discrete: ∃r >0 ∀x ∈Rd :|Λ∩Br(x)| ≤1 relatively dense: ∃R >0: ΛBR(0) =Rd

periodic with d linearly independent periods

“pure point diffractive”

we are interested in “ordered” point sets which generalise lattices I: order beyond periodicity

II: cut-and-project sets: geometry and combinatorics III: cut-and-project sets: diffraction and harmonic analysis

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order beyond periodicity motivation

### some classes of point sets

Here uniformly discrete point sets Λ. Then uniformly inx

|Λ∩Bs(x)|=O(vol(Bs)) (s → ∞) Λ Delone ⇐⇒ Λ relatively dense, uniformly discrete examples: lattice, random distortion of lattice, tilings, . . . Λ Meyer ⇐⇒Λ relatively dense, ΛΛ−1 uniformly discrete cut-and-project sets (certain projected subsets of a lattice) Meyer sets are highly structured

ΛΛ−1 uniformly discrete: finitely many “local configurations”

any Meyer set is a subset of a cut-and-project set (Meyer 72) diffraction of Delone sets: Bragg peaks of high intensity Meyer, if relatively dense (Lenz–Strungaru 14)

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order beyond periodicity finite local complexity

### patterns in uniformly discrete point sets

consider (centered ball) patterns:

r-pattern of Λ centered inp∈Λ

Λ∩Br(p), p∈Λ

patterns equivalent if they agree up to translation

Λ∩Br(p)∼Λ∩Br(q)⇐⇒p−1Λ∩Br(0) =q−1Λ∩Br(0)

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order beyond periodicity finite local complexity

### pattern counting and finite local complexity

count patterns

NB(Λ) =|{p−1Λ∩B|p ∈Λ}|

interested in (exponential) growth of NB(Λ) withB Definition

Λfinite local complexity (FLC) if NB(Λ)is finite for every ball B only finitely many “local configurations”

examples: Meyer sets, cut-and-project sets

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order beyond periodicity repetitivity

### repetitivity

we are interested in Λ with “many equivalent patterns”

Definition

Λis repetitive if∀r∃R =R(r):

Every R-ball contains an equivalent copy of every r -pattern.

For given r, one is interested in the smallest R(r)

The above condition means: ∀r∃R :∀x∈Rd∀p ∈Λ∃p0 ∈Λ:

Br(p0)⊂BR(x), Λ∩Br(p0)∼Λ∩Br(p) Λ repetitive =⇒Λ has FLC

FLC does not imply repetitivity: Z\ {0}

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order beyond periodicity repetitivity

### repetitivity function r 7→ R(r )

periodic point sets are repetitive, e.g. R(r) =r+ 1 for Λ =Z. slow growth of R(r) withr implies periodicity

Theorem (Lagarias–Pleasants 02)

LetΛbe non-empty and uniformly discrete. Assume that there exist r >0 and R(r)< 43r such that every R-ball contains an equivalent copy of every r -pattern. ThenΛis periodic.

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order beyond periodicity repetitivity

Assume w.l.o.g. 0∈Λ and define, with the above r, Pr ={p ∈Λ|Λ∩Br(p)∼Λ∩Br(0)}

Br/3(p)

p∈Pr coversRd:

repetitivity: ∀x ∈Rd∃p0 ∈Pr such thatBr(p0)⊂BR(x).

Hence d(x,p0)≤R−r <r/3.

in particularPr ∩Br/3(x)6=∅for all x ∈Rd

Pr ∩B2r/3(0) containsd linearly independent vectors:

Let x1, . . . ,xk ∈Pr ∩B2r/3(0) be linearly independent. Every r/3-ball which intersectshx1, . . . ,xki only in 0 contains some linearly independent xk+1∈Pr.

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order beyond periodicity repetitivity

Lemma

x∈Pr ∩B2r/3(0)is a period, i.e., px ∈Λ for every p∈Λ.

Proof.

Forq ∈Pr we have

i) p ∈Λ∩Br(0)⇒pq ∈Λ by definition of Pr

ii) p ∈Λ∩Br/3(q)⇒px∈Λ:

q−1pΛBr/3(0) by definition ofPr

henceq−1px ΛBr(0) byi) hencepx Λ by definition ofPr

The lemma follows since Br/3(q)

q∈Pr covers Λ.

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order beyond periodicity repetitivity

Lemma

x∈Pr ∩B2r/3(0)is a period, i.e., px ∈Λ for every p∈Λ.

Proof.

Forq ∈Pr we have

i) p ∈Λ∩Br(0)⇒pq ∈Λ by definition of Pr

ii) p ∈Λ∩Br/3(q)⇒px∈Λ:

q−1pΛBr/3(0) by definition ofPr

henceq−1px ΛBr(0) byi) hencepx Λ by definition ofPr

The lemma follows since Br/3(q)

q∈Pr covers Λ.

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order beyond periodicity examples

### Beatty sequences (Morse–Hedlund 38)

For irrationalα∈(0,1) define

bn=b(n+ 1)αc − bnαc ∈ {0,1}

repetitivity properties of Λα ={n∈Z|bn= 1}

for every r there exists finiteR(r)

Let g :R+→R+ be any continuous non-decreasing function.

Then there is α such that for infinitely manyr ∈Nthe sequence Λα cannot satisfy

R(r)≤g(r)

Any Λα arises naturally from a cut-and-project construction.

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order beyond periodicity examples

### Fibonacci substitution

a,ab,aba,abaab,abaababa,abaababaabaab, . . . 01-sequence bya7→1,b7→0.

substitution matrix:

M =

1 1 1 0

eigenvalues of M:

τ = 1 +√

5

2 = 1.618034 τ0 = 1−√ 5

2 =−0.618034. . .

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order beyond periodicity examples

### letter frequencies

n-fold substitution: no lettersMne1

Mn=

fn+1 fn

fn fn−1

Fibonacci numbers: (fn)n∈N0= 0,1,1,2,3,5,8,13,21,44, . . . fn=fn−1+fn−2= τn−τ0n

τ −τ0 =round τn

τ−τ0

relative frequency (hn(a))n∈N of aconverges:

hn(a) = fn+1

fn+1+fn = fn+1 fn+2 → 1

τ (n → ∞)

irrational limit, hence Fibonacci chain not periodic!

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order beyond periodicity examples

### Ammann–Beenker substitution

isosceles triangle (side lengths 1 and√ 2) 45-rhombus (side lengths 1)

## ↓ ↓

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order beyond periodicity examples

inflation has fix point!

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order beyond periodicity examples

### Ammann–Beenker tiling

non-periodic tiling of the plane, non-periodic vertex point set

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order beyond periodicity examples

### pinwheel tiling

triangle of side lengths 1, 2,√ 5

discovered by Conway and Radin 94 triangle orientations dense in S1 hence not FLC w.r.t. translations

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order beyond periodicity examples

### some larger patch ...

Federation Square Melbourne, Australia(Paul Bourke)

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order beyond periodicity examples

### linearly repetitive examples

study lineary repetitive point sets: R(r) =O(r) asr → ∞ there is an abundance of such point sets:

Theorem (Solomyak 97)

LetT be a substitution tiling with primitive substitution matrix. If T has finite local complexity, then T is linearly repetitive.

Here FLC and repetitivity are defined on “tile configurations”.

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order beyond periodicity uniform pattern frequencies

### pattern frequencies

box decompositions (Lagarias–Pleasants 03) box B =

### ×

d i=1

[ai,bi),vol(B)>0 box decomposition (Bi)i of B

B=[

i

Bi, Bi ∩Bj =∅ (i 6=j) B(U) set of squarish U-boxes, i.e.,bi−ai ∈[U,2U] Every box in B(W) admits decomposition in boxes from B(U), if W ≥U

B=S

U>0B(U) set of squarish boxes

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order beyond periodicity uniform pattern frequencies

### pattern counting function

wΛ(B) =|Λ∩B| boundedness: ∃C∀B ∈ B

wΛ(B)≤Cvol(B)

additivity: for every box decomposition (Bi)i of B wΛ(B) =X

i

wΛ(Bi) covariance: ∀x ∈Rd∀B ∈ B

w(xB) =wΛ(B) invariance: ∀B,B0∈ B

Λ∩B = Λ∩B0 =⇒wΛ(B) =wΛ(B0)

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order beyond periodicity uniform pattern frequencies

### pattern frequencies

upper and lower frequencies on squarishU-boxes f+(U) = sup

wΛ(B)

vol(B)|B ∈ B(U)

f(U) = inf

wΛ(B)

vol(B)|B ∈ B(U)

finite due to boundedness behaviour for U → ∞?

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order beyond periodicity uniform pattern frequencies

Theorem (Lagarias–Pleasants 03)

i) f+(U) decreases to a finite limit f as U → ∞.

ii) IfΛ is linearly repetitive, then

U→∞lim f(U) = lim

U→∞f+(U) =f

iii) IfΛ is linearly repetitive, then for every box sequence(Bn)n∈N

n→∞lim

wΛ(Bn) vol(Bn)=f,

and this convergence is uniform in the center of the boxes.

note: same result for arbitrary pattern frequencies

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order beyond periodicity uniform pattern frequencies

### proof of i)

FixW ≥U, choose B∈ B(W) and a box decomposition (Bi)i of B with boxes fromB(U).

Then by additivity wΛ(B) vol(B) =X

i

wΛ(Bi)

vol(Bi) ·vol(Bi)

vol(B) ≤f+(U) As B ∈ B(W) was arbitrary, we conclude

0≤f+(W)≤f+(U), and the claim follows by monotonicity.

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order beyond periodicity uniform pattern frequencies

### proof of ii) a la Damanik–Lenz 01

indirect proof: assume lim inf

U f(U)<lim

U f+(U) =f

Then there are many big boxes with small frequencies:

There is ε >0 andBUk ∈ B(Uk) such thatUk → ∞and wΛ(BUk)

vol(BUk) ≤f −ε

Due to linear repetitivity, such boxes will reduce the limiting value of the upper frequencyf!

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order beyond periodicity uniform pattern frequencies

### proof of ii)

Choose constant K of linear repetitivity and take arbitrary B ∈ B(3KUk).

By partitioning each side of B into 3 parts of equal length, B can be decomposed into 3d equivalent smaller boxes, each belonging to B(KUk). Denote byB(i)∈ B(KUk) the box which does not touch the boundary of B.

By linear repetitivity, there exists x∈Rd such that B0 =xBUk ⊂B(i) andx(Λ∩BUk) = Λ∩B0.

Using B ∈ B(3KUk) andB0 ∈ B(Uk), we may estimate vol(B0)

vol(B) ≥ Ukd

(2·3KUk)d = 1 (6K)d

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order beyond periodicity uniform pattern frequencies

### proof of ii)

Choose a box decomposition (Bi)ni=0 ofB, with Bi ∈ B(Uk) for i ∈ {1, . . . ,n} and estimate

wΛ(B) vol(B) =

n

X

i=1

wΛ(Bi)

vol(B) +wΛ(B0) vol(B) =

n

X

i=1

wΛ(Bi)

vol(B) +wΛ(BUk) vol(B)

n

X

i=1

f+(Uk)vol(Bi)

vol(B) + (f ε)vol(B0) vol(B)

f+(Uk) + f f+(Uk)vol(B0) vol(B) ε

(6K)d

f+(Uk) ε (6K)d

SinceB ∈ B(3KUk) was arbitrary, we have f+(3KUk)≤f+(Uk)−ε/(6K)d, a contradiction fork → ∞.

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order beyond periodicity uniform pattern frequencies

### proof of iii)

Let (Bn)n∈N any box sequence inB of diverging inradius. Since Bn∈ B(Un) for some Un, we have the estimate

f(Un)≤ wΛ(Bn)

vol(Bn) ≤f+(Un).

SinceUn→ ∞ as n→ ∞, this yields the claimed uniform convergence.

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order beyond periodicity uniform pattern frequencies

### from boxes to balls

The above result also holds for sequences of balls (Dn)n∈N of diverging radius.

tile n–ball Dn by√

n–boxes Bi n, some of which may protude the boundary ofDn

with additivity we get wΛ(Dn) vol(Dn) =X

i

wΛ(Bi n)

vol(Bi n) ·vol(Bi n) vol(Dn) result follows from uniform convergence on boxes boundary boxes are asymptotic irrelevant since

vol(Dn∂Dn)

vol(Dn) →0 (n → ∞)

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order beyond periodicity dynamical systems

### outlook: point set dynamical systems

dynamical system naturally associated to point set identify Λ with Dirac measure P

p∈Λδp

vague topology on collection of uniformly discrete point sets compact hull XΛ={xΛ|x ∈Rd}

topological dynamical system, continuous translation action Proposition

LetΛbe FLC Delone. Then Λ repetitive⇐⇒XΛ minimal.

Λ has uniform pattern frequencies⇐⇒XΛ uniquely ergodic.

Analogous result for Delone sets of infinite local complexity!

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order beyond periodicity dynamical systems

### references

J.C. Lagarias and P.A.B. Pleasants,Local complexity of Delone sets and crystallinity, Canad. Math. Bull. 45 (2002), 634–652.

J.C. Lagarias and P.A.B. Pleasants,Repetitive Delone sets and quasicrystals, Ergodic Theory Dynam. Systems 23 (2003), 831–867.

D. Damanik and D. Lenz,Linear repetitivity. I. Uniform subadditive ergodic theorems and applications, Discrete Comput. Geom. 26 (2001), 411–428.

P. M¨uller and C. Richard,Ergodic properties of randomly coloured point sets, Canad. J. Math. 65 (2013), 349–402.

D. Frettl¨oh and C. Richard,Dynamical properties of almost repetitive Delone sets, Discrete and Cont. Dynamical Systems A 34 (2014), 533–558.

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