Dependent percolation: some examples and multi-scale tools · 2020. 4. 21. · II. Oriented...

Dependent percolation:some examples and multi-scale tools

Maria Eulalia Vares

UFRJ, Rio de Janeiro, Brasil

8th Purdue International Symposium, June 2012

I. Motivation

Classical Ising model (spins ±1) in two dimensions

H(σ) = −∑

|x−y|=1

J{x,y}σxσy

σ ∈ {−1, +1}Λ; Λ ⊂ Z2 (finite)

J{x,y} ≡ J > 0 ferromagnetic interaction; x, y ∈ Z2

µΛ(σ) =1

exp {−βHΛ(σ)} (probability measure)

Phase transition at sufficiently low temperature

βc =1

2Jlog(1 +

multiple limits of µΛ as Λ → Z2 if β > βc.

Peierls (1936); Onsager (1944); Yang (1952)

I. Motivation

Simulations by Vincent Beffara

I. Motivation

Simulations by Vincent Beffara

I. Motivation

McCoy and Wu (1968) investigated the effect of random impurities

I. Motivation

Disordered ferromagnets – randomly layered environment

I. Motivation/Plan

Phase transitions for systems in a (randomly) layered environment

• Growth processes in random environment

• Some forms of coordinate percolation. Winkler’s compatibility problem.

I. Motivation/Plan

Common basic tool: multi-scale analysis

One “learns” with simpler hierarchical structures

I. Motivation/Plan

Common basic tool: multi-scale analysis

One “learns” with simpler hierarchical structures

Based on joint results with H. Kesten, B. Lima, V. Sidoravicius

II. Oriented percolation in a randomly layered environment

On Z2+ := {(x, y) ∈ Z× Z+ : x + y is even}

consider the following oriented (NW, NE) site percolation model:

quad10

consider the following oriented (NW, NE) site percolation model:

• lines Hi := {(x, y) ∈ Z2+ : y = i} are declared bad or good with probabilities δ and

1− δ respectively, independently of each other.

quad11

consider the following oriented (NW, NE) site percolation model

• Sites on good lines are open with probability pG and sites on bad lines are open with

probability pB, all independently of each other.

quad12

Regime of interest: 0 < pB < pc < pG, 0 < δ < 1.

What can we say about occurrence of percolation? (a.s. in the environment...)

quad13

Natural Guess:

δ large ⇒ no percolation

δ small ⇒ percolation

quad14

Natural Guess:

δ large ⇒ no percolation (easy)

δ small ⇒ percolation

quad15

quad16

quad17

C+(0) = {v : ∃ open oriented path from 0 to v}

Let Θ(pG, pB, δ) = P(C+(0) is infinite ).

Theorem (Kesten, Sidoravicius, V.)

∀pG > pc, ∀pB > 0, ∃δ0 > 0 so that Θ(pG, pB, δ) > 0 if δ ≤ δ0. In fact

P(C+(0) is infinite |ξ) > 0 a.s. in ξ (ξ configuration of lines)

quad18

Basic tool: multi-scale analysis

Get started with a very simple situation:

hierarchical model, L large (depending on pG, pB),

ζj :=

{k, if Lk|j but Lk+1 - j ,

0, if L - j.

Replace each entry ζj = k by k consecutive bad lines (shift the rest to the right)

• bad walls of thickness k: k consecutive bad lines;

• such bad walls at distance of order Lk from each other.

quad19

This immediately calls to the consideration of rescaled lattices, all similar to the original

one, and adapted to deal with bad lines of thickness k.

(c suitable, depending on pG)

quad20

(c suitable, depending on pG)

pG > p∗ so that P(event in the picture|seed) ≥ 1− (1− pG)2

quad21

Renormalized k-sites Sk(i,j), (i, j) ∈ Z2

quad22

Recursively define the notion of a good k-site Sk being passable from a (k − 1)-seed.

quad23

It should:

(a) guarantee the existence of open oriented paths that cross it from bottom to top

(b) produce new (k − 1)-seeds on the top,

so that

the existence of oriented k-passable paths implies the existence of open oriented paths at

scale 0, and can essentially be compared with independent site percolation.

quad24

It should:

so that

scale 0, and can essentially be compared with independent site percolation. Want estimate

pk ≤ P(

is s-passable|(k − 1)− seed)

, k ≥ 1, (∗)

quad25

It should:

so that

scale 0, and can essentially be compared with independent site percolation. Want estimate

pk ≤ P(

is s-passable|(k − 1)− seed)

, k ≥ 1, (∗)

Proposition. Given pB > 0 and pG > p∗, there exists L large enough such that (*)

holds with pk = 1− qk and

qk ≤ q2k−1 for all k ≥ 1,

and where q0 = 1− pG.

quad26

The above estimate clearly implies that for L large enough

P (there exists an infinite cluster starting from the origin) ≥+∞∏

p3k > 0.

• The main difficulty in pushing the estimate at each step is when one faces the bad

wall of larger mass.

• Planarity plays important role in these arguments.

• Enlarging the seeds and taking some extra care replace p∗ by pc.

quad27

bad layer of mass k

(i, j)

SS(i+1, j-1)

(i-1, j-1)

quad28

bad line of mass k-1

reverse partition

forward partition

S(i, j)

S(i, j+1)

(i, j+1)

bad line of mass k-2

quad29

Dealing with random layers

• Step 1: Devise a suitable grouping procedure

• Step 2: Perform the recursive (much more involved!) estimates

quad30

Step 1: Grouping procedure

ξ ∈ {0, 1}N sampled from the Bernoulli distribution Pδ with low density δ.

Γ = {i : ξi = 1} - correspond to the “bad lines” (of level 0):

L ≥ 3 integer.

quad31

L ≥ 3 integer. Start with L-runs ...

quad32

quad33

Assuming 64δL2 < 1 the procedure converges:

Each x ∈ Γ will be “re-incorporated” finitely many times a.s.; the final partition is

well defined.

quad34

On a set Ξ(δ) of full measure we can decompose Γ into sets Ci, called clusters, to which

an N-valued mass m(Ci) is attributed (m(Ci) ≤ |Ci|) in a way that

d(Ci, Cj) ≥ Lmin{m(Ci),m(Cj)}

, for all i, j

The Ci := C∞,i are obtained by the (limiting) recursive procedure outlined above. Each

constructed cluster has a level (the step when it was born!) and a mass.

m m1 2 3

m1 m2 m3+ + _ _ logLd1 logL d2

quad35

Step 2: Multi-scale analysis for fixed realization of good/bad lines

Assume 3 ≤ L, 64δL2 < 1.

χ(ξ) := inf{k ≥ 0: d(C, 0) ≥ Mm(C)

for all C ∈ C∞ with m(C) > k}

with χ(ξ) = ∞ if the above set is empty or ξ /∈ Ξ(δ)

Pδ{ξ : χ(ξ) < ∞} = 1 and Pδ{ξ : χ(ξ) = 0} > 0.

We prove

C+(0) is infinite)|χ(ξ) < ∞

Conceptually, the structure is similar to that in the simple hierarchical situation, but:

• rescaled lattices depend also on ξ;

• main estimate (drilling through the higher mass) within a good k-site Sk is much more

involved; our estimates require L somehow larger (L ≥ 192 suffices);

• pk ↗ 1 exponentially in k.

quad36

III. Some related results

Bramson, Durrett, Schonmann (1991)

quad37

III. Some related results

Bramson, Durrett, Schonmann (1991)

quad38

IV. Coordinate percolation. Winkler’s compatibility

(η, ξ) a pair of sequences in Ξ = {0, 1}N

Allowed to: remove ones from η; remove zeros from ξ

Can one map both sequences to the same semi-infinite sequence?

If YES, say that (η, ξ) is compatible.

quad39

Winkler’s compatibility question:

Does it exist (p′, p) ∈ (0, 1)2 such that

Pp′ ⊗ Pp{(η, ξ) ∈ Ξ× Ξ: (η, ξ) are compatible} > 0 ?

P. Gacs (2004). Recent preprints: Basu, Sly (2012), Sidoravicius (2012).

quad40

Winkler’s compatibility question:

Does it exist (p′, p) ∈ (0, 1)2 such that

Pp′ ⊗ Pp{(η, ξ) ∈ Ξ× Ξ: (η, ξ) are compatible} > 0 ?

P. Gacs (2004). Recent preprints: Basu, Sly (2012), Sidoravicius (2012).

Let p ∈ (0, 1). Say that η ∈ Ξ is p-compatible if

Pp{ξ ∈ Ξ: (η, ξ) is compatible} > 0.

quad41

Theorem (Kesten, Lima, Sidoravicius, V.)

For every ε > 0 there exist 0 < pε < 1 and a binary sequence η ≡ ηε ∈ Ξ, such

that Zηε is a discrete fractal with Hausdorff dimension dH(Zη) ≥ 1− ε, and such that

Pp{ξ ∈ Ξ: (η, ξ) is compatible} > 0

for any p < pε.

Notation: Zη = {i ≥ 1: ηi = 0}

For the proof

• exploit a representation as coordinate oriented percolation;

• essential ingredient: the grouping procedure mentioned before.

Move from ξ to ψ ∈ ZN+: ψi ≥ 1 representing the length of the corresponding run of 1s.

ψ ∈ ZN+ : ψi ≥ 1 implies ψi+1 = 0}

quad42

Coordinate percolation process.

Oriented graph G = (V,E), where

V = Z2+ E = { vertical n.n., northeast diagonals } oriented upwards

Given ζ, ψ ∈ Ψ, define the site configuration ωζ,ψ on G: for v = (v1, v2) with

v1, v2 ≥ 1

ωζ,ψ(v) =

{1 if ζv1

≥ ψv2,

0 otherwise.

v ∈ V open iff ωζ,ψ(v) = 1

(ωζ,ψ(0, 0) = 1, ωζ,ψ(v) = 0 if v1 ∧ v2 = 0)

quad43

Simulations of coordinate percolation (by Lionel Levine)

quad44

quad45

quad46

quad47

quad48

quad49

��

Figure 1: oriented cluster of the origin

For the compatibility question, we need more than an open oriented path.

A vertex v = (v1, v2) with v1, v2 ≥ 1 is heavy if ψv2≥ 1.

quad50

Permitted path: does not cross two heavy vertices with the same first coordinate.

��

Figure 2: open permitted path from the origin

quad51

Let ζ, ψ ∈ Ψ. If there exists an infinite open permitted path π starting from the origin

for the percolation configuration ωζ,ψ, then the pair (ζ, ψ) is compatible.

quad52

M -spaced sequences

M ≥ 2 integer. A sequence ψ ∈ Ψ is M -spaced if:

a) ij(ψ) ≥ M j for all j ≥ 1, where

ij(ψ) = inf{n ∈ N : ψn ≥ j} (+∞ if { } = ∅)

b) j − i ≥ Mmin{ψi,ψj}, for all 1 ≤ i < j.

ΨM := {ξ ∈ Ψ: ξ is M -spaced}

quad53

M -spaced sequences

M ≥ 2 integer. A sequence ψ ∈ Ψ is M -spaced if:

a) ij(ψ) ≥ M j for all j ≥ 1, where

ij(ψ) = inf{n ∈ N : ψn ≥ j} (+∞ if { } = ∅)

b) j − i ≥ Mmin{ψi,ψj}, for all 1 ≤ i < j.

ΨM := {ξ ∈ Ψ: ξ is M -spaced}

Theorem

Let L ≥ 2 and M ≥ 3(L + 1) be integers, ζ(L) the hierarchical sequence as before,

and ψ ∈ ΨM . Then the configuration ωζ(L),ψ has an infinite open permitted path π

starting from the origin.

Corollary

If ψ ∈ ΨM with M = 3(L + 1) the pair (ζ(L), ψ) is compatible.

quad54

Using this and the grouping lemma discussed before, one gets:

Theorem

Let L ≥ 2 and M = 3(L + 1). If p < 164M2 , ζ(L) ∈ Ψ is given by

(ζ(L))j =

{3Mk−1, if Lk|j and Lk+1 - j,0 , if L - j,

and η(L) is the corresponding binary sequence, then

Pp{ξ ∈ Ξ: (η(L), ξ) is compatible} > 0.

Remark. The statement about the zero set of η(L) is simple to verify, by classical results

(Barlow and Taylor).

quad55

V. Related problems

Winkler’s Clairvoyant Demon problem

quad56

V. Related problems

quad57

V. Related problems

quad58

V. Related problems

quad59

V. Related problems

quad60

V. Related problems

1 2 nr

Formulation as oriented percolation problem - Noga Alon

quad61

V. Related problems

1 2 nr

Formulation as oriented percolation problem - Noga Alon

quad62

V. Related problems

Negative answer if n = 2 or n = 3;

n ≥ 4 - open question

Positive answer in the unoriented case for n ≥ 4: P Winkler (2000); Balister, Bollobas,

Stacey (2000)

For the oriented model: Gacs (2000): The Clairvoyant Demon has a hard task ...

quad63

V. Related problems

- Stretched lattices: Jonasson, Mossel, Peres ; Hoffman

- Unoriented percolation; Potts model: Kesten, Lima, Sidoravicius, V.

- Percolation of words: Grimmett, Liggett, Richthammer (2008), Lima (2008, 2009)

- Rough isometries: Peled (2010), recent work: Basu, Sly, Sidoravicius.

quad64

References

On the compatibility of binary sequences. H. Kesten, B. Lima, V. Sidoravicius, M.E.

Vares. (preprint on arXiv)

Oriented percolation in a random environment. H. Kesten, V. Sidoravicius, M.E. Vares.

(preprint on arXiv)

Dependent percolation on Z2. H. Kesten, B. Lima, V. Sidoravicius, M.E. Vares. (preprint)

Lipschitz embeddings of random sequences. R. Basu, A. Sly. (preprint on arXiv)

quad65

Dependent percolation: some examples and multi-scale tools · 2020. 4. 21. · II. Oriented...

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