Permutations and permutation graphs

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Permutations and permutation graphs Robert Brignall Schloss Dagstuhl, 8 November 2018 1 / 25

Transcript of Permutations and permutation graphs

Page 1: Permutations and permutation graphs

Permutations and permutation graphs

Robert Brignall

Schloss Dagstuhl, 8 November 2018

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Page 2: Permutations and permutation graphs

Permutations and permutation graphs

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• Permutation π = π(1) · · ·π(n)• Inversion graph Gπ: for i < j, ij ∈ E(Gπ) iff π(i) > π(j).• Note: n · · · 21 becomes Kn.• Permutation graph = can be made from a permutation

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Permutations and permutation graphs

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• Permutation π = π(1) · · ·π(n)• Inversion graph Gπ: for i < j, ij ∈ E(Gπ) iff π(i) > π(j).• Note: n · · · 21 becomes Kn.• Permutation graph = can be made from a permutation

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Ordering permutations: containment

1 3 5 2 4 < 4 1 2 6 3 8 5 7

• ‘Classical’ pattern containment: σ ≤ π.• Translates to induced subgraphs: Gσ ≤ind Gπ.• Permutation class: a downset:

π ∈ C and σ ≤ π implies σ ∈ C.

• Avoidance: minimal forbidden permutation characterisation:

C = Av(B) = {π : β 6≤ π for all β ∈ B}.3 / 25

Page 5: Permutations and permutation graphs

Ordering permutations: containment

1 3 5 2 4 < 4 1 2 6 3 8 5 7

• ‘Classical’ pattern containment: σ ≤ π.• Translates to induced subgraphs: Gσ ≤ind Gπ.• Permutation class: a downset:

π ∈ C and σ ≤ π implies σ ∈ C.

• Avoidance: minimal forbidden permutation characterisation:

C = Av(B) = {π : β 6≤ π for all β ∈ B}.3 / 25

Page 6: Permutations and permutation graphs

Ordering permutations: containment

1 3 5 2 4 < 4 1 2 6 3 8 5 7

• ‘Classical’ pattern containment: σ ≤ π.• Translates to induced subgraphs: Gσ ≤ind Gπ.• Permutation class: a downset:

π ∈ C and σ ≤ π implies σ ∈ C.

• Avoidance: minimal forbidden permutation characterisation:

C = Av(B) = {π : β 6≤ π for all β ∈ B}.3 / 25

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Ordering graphs: induced subgraphs

• Induced subgraph: H ≤ind G: ‘delete vertices’ (& incident edges).• Hereditary class: C, a downset:

G ∈ C and H ≤ind G =⇒ H ∈ C.

(Example: all planar graphs.)• Forbidden induced subgraph characterisation, Free(G1, . . . , Gk).

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Ordering graphs: induced subgraphs

≤ind

• Induced subgraph: H ≤ind G: ‘delete vertices’ (& incident edges).• Hereditary class: C, a downset:

G ∈ C and H ≤ind G =⇒ H ∈ C.

(Example: all planar graphs.)• Forbidden induced subgraph characterisation, Free(G1, . . . , Gk).

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Dictionary

Permutations GraphsPermutation π Permutation graph Gπ

Containment π < σ Induced subgraph Gπ <ind Gσ

Class C Class GC = {Gπ : π ∈ C}.

Av(321) Bipartite permutation graph

Av(231) Free(C4, P4)

Av(312) ”

Av(231, 312) Free( )

Av(2413, 3142)(separables) Cographs: Free(P4)

Av(3412, 2143) (skew-merged) Split permutation graphs:Free(2K2, C4, C5, S3, rising sun,net, rising sun)

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Dictionary

Permutations GraphsPermutation π Permutation graph Gπ

Containment π < σ Induced subgraph Gπ <ind Gσ

Class C Class GC = {Gπ : π ∈ C}.

Av(321) Bipartite permutation graph

Av(231) Free(C4, P4)

Av(312) ”

Av(231, 312) Free( )

Av(2413, 3142)(separables) Cographs: Free(P4)

Av(3412, 2143) (skew-merged) Split permutation graphs:Free(2K2, C4, C5, S3, rising sun,net, rising sun)

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Dictionary

Permutations GraphsPermutation π Permutation graph Gπ

Containment π < σ Induced subgraph Gπ <ind Gσ

Class C Class GC = {Gπ : π ∈ C}.

Av(321) Bipartite permutation graph

Av(231) Free(C4, P4)

Av(312) ”

Av(231, 312) Free( )

Av(2413, 3142)(separables) Cographs: Free(P4)

Av(3412, 2143) (skew-merged) Split permutation graphs:Free(2K2, C4, C5, S3, rising sun,net, rising sun)

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Dictionary

Permutations GraphsPermutation π Permutation graph Gπ

Containment π < σ Induced subgraph Gπ <ind Gσ

Class C Class GC = {Gπ : π ∈ C}.

Av(321) Bipartite permutation graph

Av(231) Free(C4, P4)

Av(312) ”

Av(231, 312) Free( )

Av(2413, 3142)(separables) Cographs: Free(P4)

Av(3412, 2143) (skew-merged) Split permutation graphs:Free(2K2, C4, C5, S3, rising sun,net, rising sun)

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Permutation graphs as a graph class

Graphclasses.org tells me that:

Perm. graphs = Free(Cn+4, T2, X2, X3, X30, X31, X32, X33, X34, X36,

XF2n+31 , XFn+1

2 , XFn3 , XFn

4 , XF2n+35 , XF2n+2

6 ,+ complements)

N.B. (e.g.) Cn+4 are all the cycles of length ≥ 5, so this is an infinite list.

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Today

Two interactions between permutations and graphs

1. Clique width2. Labelled well-quasi-ordering

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§1 Clique width

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Build-a-graph

Set of labels Σ. You have 4 operations to build a labelled graph:1. Create a new vertex with a label i ∈ Σ.2. Disjoint union of two previously-constructed graphs.3. Join all vertices labelled i to all labelled j, where i, j ∈ Σ, i 6= j.4. Relabel every vertex labelled i with j.

Example (Binary trees need at most 3 labels)

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Build-a-graph

Set of labels Σ. You have 4 operations to build a labelled graph:1. Create a new vertex with a label i ∈ Σ.2. Disjoint union of two previously-constructed graphs.3. Join all vertices labelled i to all labelled j, where i, j ∈ Σ, i 6= j.4. Relabel every vertex labelled i with j.

• Clique-width, cw(G) = size of smallest Σ needed to build G.• If H ≤ind G, then cw(H) ≤ cw(G).• Clique-width of a class C

cw(C) = maxG∈C

cw(G)

if this exists.

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Motivation

Theorem (Courcelle, Makowsky and Rotics (2000))

If cw(C) < ∞, then any property expressible in monadic second-order(MSO1) logic can be determined in polynomial time for C.

• MSO1 includes many NP-hard algorithms: e.g. k-colouring(k ≥ 3), graph connectivity, maximum independent set,. . .

• Generalises treewidth, critical to the proof of the Graph MinorTheorem (see next slide)

• Unlike treewidth, clique-width can cope with dense graphs

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Diversion: treewidth, tw(G)

• tw(G) measures ‘how like a tree’ G is (tw(G) = 1 iff G is a tree).• Bounded treewidth =⇒ all problems in MSO2 in polynomial time.

Theorem (Robertson and Seymour, 1986)

For a minor-closed family of graphs C, tw(C) bounded if and only if C doesnot contain all planar graphs.

• Planar graphs are the unique “minimal” family for treewidth.

QuestionCan we get a similar theorem for clique width?

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Yes! For vertex-minors. . .

• Vertex-minor = induced subgraph + local complements

Theorem (Geelen, Kwon, McCarty, Wollan (announced 2018))A vertex-minor-closed downset of graphs has unbounded clique-width if andonly if it contains every circle graph as a vertex-minor.

Circle graph = intersection graph of chords

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Yes! For vertex-minors. . .

• Vertex-minor = induced subgraph + local complements

Theorem (Geelen, Kwon, McCarty, Wollan (announced 2018))A vertex-minor-closed downset of graphs has unbounded clique-width if andonly if it contains every circle graph as a vertex-minor.

Circle graph = intersection graph of chords

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Yes! For vertex-minors. . .

• Vertex-minor = induced subgraph + local complements

Theorem (Geelen, Kwon, McCarty, Wollan (announced 2018))A vertex-minor-closed downset of graphs has unbounded clique-width if andonly if it contains every circle graph as a vertex-minor.

Circle graph = intersection graph of chords

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Yes! For vertex-minors. . .

• Vertex-minor = induced subgraph + local complements

Theorem (Geelen, Kwon, McCarty, Wollan (announced 2018))A vertex-minor-closed downset of graphs has unbounded clique-width if andonly if it contains every circle graph as a vertex-minor.

Circle graph = intersection graph of chords

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Yes! For vertex-minors. . .

• Vertex-minor = induced subgraph + local complements

Theorem (Geelen, Kwon, McCarty, Wollan (announced 2018))A vertex-minor-closed downset of graphs has unbounded clique-width if andonly if it contains every circle graph as a vertex-minor.

Circle graph = intersection graph of chords

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Clique width on graph classes

Bounded clique-width• Cographs, C = Free(P4): cw(C) = 2.• F = {forests}: cw(F ) = 3.

Unbounded clique-width• Circle graphs• Split permutation graphs• Bipartite permutation graphs• Any class with superfactorial speed

(∼more than ncn labelled graphs of order n, for any c)

QuestionWhat are the minimal classes of graphs with unbounded clique-width?

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Clique width on graph classes

Bounded clique-width• Cographs, C = Free(P4): cw(C) = 2.• F = {forests}: cw(F ) = 3.

Unbounded clique-width• Circle graphs• Split permutation graphs• Bipartite permutation graphs• Any class with superfactorial speed

(∼more than ncn labelled graphs of order n, for any c)

QuestionWhat are the minimal classes of graphs with unbounded clique-width?

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Clique width on graph classes

Bounded clique-width• Cographs, C = Free(P4): cw(C) = 2.• F = {forests}: cw(F ) = 3.

Unbounded clique-width• Circle graphs• Split permutation graphs• Bipartite permutation graphs• Any class with superfactorial speed

(∼more than ncn labelled graphs of order n, for any c)

QuestionWhat are the minimal classes of graphs with unbounded clique-width?

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Clique width on graph classes

Bounded clique-width• Cographs, C = Free(P4): cw(C) = 2.• F = {forests}: cw(F ) = 3.

Unbounded clique-width• Circle graphs• Split permutation graphs ←−minimal!• Bipartite permutation graphs ←−minimal!• Any class with superfactorial speed

(∼more than ncn labelled graphs of order n, for any c)

QuestionWhat are the minimal classes of graphs with unbounded clique-width?

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Permutations as circles

Circle graphs are minimal with respect to vertex minor, but not forinduced subgraphs.

Permutation graphs ⊂ Circle graphs

1

12

2

3

3

44

5

5

_ _ _ _ _

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Permutations as circles

Circle graphs are minimal with respect to vertex minor, but not forinduced subgraphs.

Permutation graphs ⊂ Circle graphs

1

12

2

3

3

44

5

5

_ _ 1 _ _

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Permutations as circles

Circle graphs are minimal with respect to vertex minor, but not forinduced subgraphs.

Permutation graphs ⊂ Circle graphs

1

12

2

3

3

44

5

5

_ _ 1 _ 2

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Permutations as circles

Circle graphs are minimal with respect to vertex minor, but not forinduced subgraphs.

Permutation graphs ⊂ Circle graphs

1

12

2

3

3

44

5

5

3 _ 1 _ 2

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Permutations as circles

Circle graphs are minimal with respect to vertex minor, but not forinduced subgraphs.

Permutation graphs ⊂ Circle graphs

1

12

2

3

3

44

5

5

3 _ 1 4 2

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Permutations as circles

Circle graphs are minimal with respect to vertex minor, but not forinduced subgraphs.

Permutation graphs ⊂ Circle graphs

1

12

2

3

3

44

5

5

3 5 1 4 2

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Av(321) vs Bipartite permutation graphs

Theorem (Lozin, 2011)Bipartite permutation graphs are a minimal class with unboundedclique-width.

Permutations Graphs

π = 321 Gπ =

Class: Av(321) Bipartite permutation

Structure:

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Av(321) vs Bipartite permutation graphs

Theorem (Lozin, 2011)Bipartite permutation graphs are a minimal class with unboundedclique-width.

Permutations Graphs

π = 321 Gπ =Class: Av(321) Bipartite permutation

Structure:

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Av(321) vs Bipartite permutation graphs

Theorem (Lozin, 2011)Bipartite permutation graphs are a minimal class with unboundedclique-width.

Permutations Graphs

π = 321 Gπ =Class: Av(321) Bipartite permutation

Structure:

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Av(321) vs Bipartite permutation graphs

Theorem (Lozin, 2011)Bipartite permutation graphs are a minimal class with unboundedclique-width.

Permutations Graphs

π = 321 Gπ =Class: Av(321) Bipartite permutation

Structure:

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Split permutation graphs

Theorem (Atminas, B., Lozin, Stacho, 2018+)Split permutation graphs are a minimal class with unbounded clique-width.

Permutations Graphs

Merge of 1 . . . k, j . . . 1 Indep set + clique

Class: Av(2143, 3412) Split permutation

Structure:

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Split permutation graphs

Theorem (Atminas, B., Lozin, Stacho, 2018+)Split permutation graphs are a minimal class with unbounded clique-width.

Permutations Graphs

Merge of 1 . . . k, j . . . 1 Indep set + cliqueClass: Av(2143, 3412) Split permutation

Structure:

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Split permutation graphs

Theorem (Atminas, B., Lozin, Stacho, 2018+)Split permutation graphs are a minimal class with unbounded clique-width.

Permutations Graphs

Merge of 1 . . . k, j . . . 1 Indep set + cliqueClass: Av(2143, 3412) Split permutation

Structure:

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Split permutation graphs

Theorem (Atminas, B., Lozin, Stacho, 2018+)Split permutation graphs are a minimal class with unbounded clique-width.

Permutations Graphs

Merge of 1 . . . k, j . . . 1 Indep set + cliqueClass: Av(2143, 3412) Split permutation

Structure:

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More minimal classes?

• Permutation class structure is a long ‘path’:

?• Could find minimal classes of permutation graphs.• Carry out local complementation to make other

(non-permutation) graph classes.

Corollary (to Geelen, Kwon, McCarty, Wollan)Every minimal class of unbounded clique width is a subclass of circlegraphs.

Question (possibly naive)Are all these classes related to each other by local complementation?

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§2 Labelled well-quasi-ordering

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Infinite labelled antichains

• Antichain: set of pairwise incomparable graphs/permutations

The set of cycles forms an infinite antichain

· · ·

Paths form a labelled infinite antichain· · ·

Increasing oscillations/Gollan permutations too...

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Page 46: Permutations and permutation graphs

Infinite labelled antichains

• Antichain: set of pairwise incomparable graphs/permutations

The set of cycles forms an infinite antichain

· · ·

Paths form a labelled infinite antichain· · ·

Increasing oscillations/Gollan permutations too...

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Page 47: Permutations and permutation graphs

Infinite labelled antichains

• Antichain: set of pairwise incomparable graphs/permutations

The set of cycles forms an infinite antichain

· · ·

Paths form a labelled infinite antichain· · ·

Increasing oscillations/Gollan permutations too...

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Page 48: Permutations and permutation graphs

Infinite labelled antichains

• Antichain: set of pairwise incomparable graphs/permutations

The set of cycles forms an infinite antichain

· · ·

Paths form a labelled infinite antichain· · ·

Increasing oscillations/Gollan permutations too...

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No labelled antichains

• well-quasi-order (WQO): no infinite antichain.• Labelled well-quasi-order (LWQO): no infinite labelled antichain.

Theorem (Pouzet, 1972)Every LWQO class (of graphs, permutations, anything) is finitely based.

Conjecture (Korpelainen, Lozin & Razgon, 2013;Atminas & Lozin, 2015)Every finitely based WQO graph class must also be LWQO.

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Intuition

Conjecture (KLR, 2013; AL, 2015)Every finitely based WQO graph class must also be LWQO.

If a graph contains long paths, then it contains

· · ·

. . . so is not LWQO.

But then, you can’t avoid

· · ·

. . . unless they are all in the basis.

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Intuition

Conjecture (KLR, 2013; AL, 2015)Every finitely based WQO graph class must also be LWQO.

If a graph contains long paths, then it contains

· · ·

. . . so is not LWQO.

But then, you can’t avoid

· · ·

. . . unless they are all in the basis.

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‘Obviously’ wrong for permutations

Increasing oscillations again

PropositionThe smallest class containing the increasing oscillations isAv(321, 2341, 3412, 4123) and is WQO (but not LWQO).

But. . .As a graph class, Cn is a basis element for n ≥ 5.⇒ not a counterexample.

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Page 53: Permutations and permutation graphs

‘Obviously’ wrong for permutations

Increasing oscillations again

PropositionThe smallest class containing the increasing oscillations isAv(321, 2341, 3412, 4123) and is WQO (but not LWQO).

But. . .As a graph class, Cn is a basis element for n ≥ 5.⇒ not a counterexample.

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Another permutation example

Proposition (B., Engen, Vatter, 2018+)

Av(2143, 2413, 3412, 314562, 412563, 415632, 431562, 512364, 512643,516432, 541263, 541632, 543162) is another WQO-but-not-LWQO class.

Here’s the labelled antichain

. . .

Corollary

The class Free(2K2, C4, C5, net, co-net, rising sun, co-rising sun, H, H, cross,co-cross, X168, X168, X160), is WQO but not LWQO.

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Another permutation example

Proposition (B., Engen, Vatter, 2018+)

Av(2143, 2413, 3412, 314562, 412563, 415632, 431562, 512364, 512643,516432, 541263, 541632, 543162) is another WQO-but-not-LWQO class.

Here’s the labelled antichain

. . .

Corollary

The class Free(2K2, C4, C5, net, co-net, rising sun, co-rising sun, H, H, cross,co-cross, X168, X168, X160), is WQO but not LWQO.

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Closing thoughts: WQO and clique-width

Conjecture (Daligault, Rao, Thomassé, 2010)If C is labelled well-quasi-ordered, then C has bounded clique-width.

N.B.WQO does not imply bounded clique width (Lozin, Razgon,Zamaraev, 2018).

. . .

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Thanks!

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