Upper bounds on small Ramsey numbersRamsey NumbersFlag AlgebraApplicationResultsExample Flag...

Ramsey Numbers Flag Algebra Application Results Example

Upper bounds on small Ramseynumbers

Bernard Lidicky Florian Pfender

Iowa State UniversityUniversity of Colorado Denver

Atlanta Lecture Series in Combinatorics and Graph Theory XIVFeb 15, 2015

DefinitionR(G1,G2, . . . ,Gk) is the smallest integer n such that any k-edgecoloring of Kn contains a copy of Gi in color i for some 1 ≤ i ≤ k .

R(K3,K3) > 5 R(K3,K3) ≤ 6

Theorem (Ramsey 1930)

R(Km,Kn) is finite.

R(G1, . . . ,Gk) is finite

Questions:

• study how R(G1, . . . ,Gk) grows if G1, . . . ,Gk grow (large)

• study R(G1, . . . ,Gk) for fixed G1, . . . ,Gk (small)

Radziszowski - Small Ramsey NumbersElectronic Journal of Combinatorics - Survey

R(Km,Kn) is finite.

Questions:

R(Km,Kn) is finite.

Questions:

Flag algebras

Seminal paper:Razborov, Flag Algebras, Journal of SymbolicLogic 72 (2007), 1239–1282.David P. Robbins Prize by AMS for Razborov in2013

Example (Goodman, Razborov)

If density of edges is at least ρ > 0, what is the minimum densityof triangles?

• designed to attack extremal problems.

• works well if constraints as well as desired value can be computedby checking small subgraphs (or average over small subgraphs)

• the results are in limit (very large graphs)

Flag algebras

Seminal paper:Razborov, Flag Algebras, Journal of SymbolicLogic 72 (2007), 1239–1282.David P. Robbins Prize by AMS for Razborov in2013

Example (Goodman, Razborov)

If density of edges is at least ρ > 0, what is the minimum densityof triangles?

• designed to attack extremal problems.

• works well if constraints as well as desired value can be computedby checking small subgraphs (or average over small subgraphs)

• the results are in limit (very large graphs)

Applications (incomplete list)Author Year Application/ResultRazborov 2008 edge density vs. triangle densityHladky, Kra ,l, Norin 2009 Bounds for the Caccetta-Haggvist conjectureRazborov 2010 On 3-hypergraphs with forbidden 4-vertex configurationsHatami, Hladky,Kra ,l,Norine,Razborov / Grzesik 2011 Erdos Pentagon problemHatami, Hladky, Kra ,l, Norin, Razborov 2012 Non-Three-Colourable Common Graphs ExistBalogh, Hu, L., Liu / Baber 2012 4-cycles in hypercubesReiher 2012 edge density vs. clique densityShagnik, Huang, Ma, Naves, Sudakov 2013 minimum number of k-cliquesBaber, Talbot 2013 A Solution to the 2/3 ConjectureFalgas-Ravry, Vaughan 2013 Turan density of many 3-graphsCummings, Kra ,l, Pfender, Sperfeld, Treglown, Young 2013 Monochromatic triangles in 3-edge colored graphsKramer, Martin, Young 2013 Boolean latticeBalogh, Hu, L., Pikhurko, Udvari, Volec 2013 Monotone permutationsNorin, Zwols 2013 New bound on Zarankiewicz’s conjectureHuang, Linial, Naves, Peled, Sudakov 2014 3-local profiles of graphsBalogh, Hu, L., Pfender, Volec, Young 2014 Rainbow triangles in 3-edge colored graphsBalogh, Hu, L., Pfender 2014 Induced density of C5Goaoc, Hubard, de Verclos, Sereni, Volec 2014 Order type and density of convex subsetsCoregliano, Razborov 2015 Tournaments... ... ...

Applications to graphs, oriented graphs, hypergraphs, hypercubes,permutations, crossing number of graphs, order types, geometry,. . . Razborov: Flag Algebra: an Interim Report

Inspiration

Theorem (Cummings, Kra ,l, Pfender, Sperfeld, Treglown, Young)

In every 3-edge-colored complete graph on n vertices, there are atleast 1

)+ o(n3) monochromatic triangles.

+ + ≥ 1

25+ o(1)

25subject to = = 0

≥ 15

Inspiration

+ + ≥ 1

+ o(1)

25subject to = = 0

≥ 15

Inspiration

+ + ≥ 1

+ o(1)

25subject to = = 0

≥ 15

Inspiration

+ + ≥ 1

+ o(1)

25subject to = = 0

≥ 15

Inspiration

+ + ≥ 1

+ o(1)

25subject to = = 0

≥ 15

Inspiration

+ + ≥ 1

+ o(1)

25subject to = = 0

≥ 15

Example

What is number of non-edges in a blow-up?

5∑i=1

(|Ii |2

5∑i=1

)≥ 5

)≈ 1

Observation (Key observation)

If a Ramsey graph G has k vertices, then the density of non-edgesin any blow-up of G is at least 1

k +o(1).

Example

5∑i=1

(|Ii |2

5∑i=1

)≥ 5

)≈ 1

k +o(1).

Example

5∑i=1

(|Ii |2

5∑i=1

)≥ 5

)≈ 1

k +o(1).

Outline of idea

k +o(1).

• Let G be 2-edge-colored complete graphs with no monochromatictriangle.

• Consider all blow-ups B of graphs in G• ∀B ∈ B, density of non-edges in B is at least 1

k = 15 .

ObservationIf density of non-edges ρ is > 1

k+1 over all B ∈ B, then Ramseygraph has as most k vertices.

If one can prove ρ > 16 , then there is no Ramsey graphs on 6

vertices.Notice that any lower bound on ρ in ( 1

k+1 ,1k ] gives Ramsey graph

has at most k vertices.

Outline of idea

k +o(1).

k = 15 .

Outline of idea

k +o(1).

k = 15 .

Outline of idea

k +o(1).

k = 15 .

vertices.

Notice that any lower bound on ρ in ( 1k+1 ,

1k ] gives Ramsey graph

Outline of idea

k +o(1).

k = 15 .

Outline of idea

k +o(1).

k = 15 .

R(G1, . . . ,Gn) ≤ 1 + 1/ρ

Blow-ups in Flag Algebra

How to characterize blow-ups B of

graphs with no , ?

Forbidden subgraphs :

, , , ,

minimize

subject to = = = = = 0

Flag Algebra question! Easy to modify.

graphs with no , ?

, , , ,

minimize

graphs with no , ?

, , , ,

minimize

graphs with no , ?

, , , ,

minimize

New upper bounds (so far)

Problem Lower New upper Old upper

R(K−4 ,K

−4 ,K

−4 ) 28 28 30

R(K3,K−4 ,K

−4 ) 21 23 27

R(K4,K−4 ,K

−4 ) 33 47 59

R(K4,K4,K−4 ) 55 104 113

R(C3,C5,C5) 17 18 21?R(K4,K

−7 ) 37 52 59

R(K2,2,2,K2,2,2) 30 32 60?R(K−

5 ,K−6 ) 31 38 39

R(K5,K−6 ) 43 62 67

Example of Computation

LemmaR(K3,K3) ≤ 6

Our goal is to show:

6subject to = = = = = 0

We show perhaps the most complicated proof of the lemma!

LemmaR(K3,K3) ≤ 6

6subject to = = = = = 0

LemmaR(K3,K3) ≤ 6

6subject to = = = = = 0

Observe that and can be swapped. Change to a colorblind

setting. is a monochromatic triangle (red or blue).

Our new goal is to show:

6subject to = = = 0

Colorblind setting will allow to fit the computation on these slides.Also important for bigger applications.

6subject to = = = = = 0

Observe that and can be swapped.

Change to a colorblind

6subject to = = = 0

6subject to = = = = = 0

6subject to = = = 0

6subject to = = = = = 0

6subject to = = = 0

6subject to = = = = = 0

6subject to = = = 0

Basic equations:

+ + + + + + = 1

(1 + 0 + 0 + 1 + 3 + 2 + 6

6subject to = = = 0

Basic equations:

+ + + + + + = 1

(1 + 0 + 0 + 1 + 3 + 2 + 6

We use flags with type σ1 of size two

For a positive semidefinite matrix M

0 ≤rFTMF

0.0744 −0.0223 −0.0520−0.0223 0.0238 −0.0014−0.0520 −0.0014 0.0536

=− 0.0116 − 0.3568 − 0.1784 − 0.0112

+ 0.3216 + 0 + 0 .

(1 + 0 + 0 + 1 + 3 + 2 + 6

0 ≥ 0.0116 + 0.3568 + 0.1784 + 0.0112

−0.3216 + 0 + 0 .

We sum the equations and obtain

≥ 0.1782 + 0.3568 + 0.1784 + 0.1778

+ 0.1784 + 0.33 + > 0.17 >1

Note that the matrix M was not unique or tight (easy rounding).(bound ≥ 1

5 is obtainable)

(1 + 0 + 0 + 1 + 3 + 2 + 6

0 ≥ 0.0116 + 0.3568 + 0.1784 + 0.0112

−0.3216 + 0 + 0 .

≥ 0.1782 + 0.3568 + 0.1784 + 0.1778

+ 0.1784 + 0.33 + > 0.17 >1

5 is obtainable)

(1 + 0 + 0 + 1 + 3 + 2 + 6

0 ≥ 0.0116 + 0.3568 + 0.1784 + 0.0112

−0.3216 + 0 + 0 .

≥ 0.1782 + 0.3568 + 0.1784 + 0.1778

+ 0.1784 + 0.33 + > 0.17 >1

5 is obtainable)

What have we tried (so far)Problem Lower Upper Our upper Graphs

R(K−4 ,K−

4 ,K−4 ) 28 30 28 2589

R(K3,K−4 ,K−

4 ) 21 27 23 4877R(K4,K

−4 ,K−

4 ) 33 59 47 9476R(K4,K4,K

−4 ) 55 113 104 11410

R(C3,C5,C5) 17 21? 18 5291R(K4,K

−7 ) 37 52 49 11747

R(K2,2,2,K2,2,2) 30 60? 32 8792R(K−

5 ,K−6 ) 31 39 38 14889

R(K5,K−6 ) 43 67 62 18186

R(K4,K6) 36 41 44 11667R(K4,K7) 49 61 67 11765R(K5,K5) 43 49 53 8722R(K5,K

−5 ) 30 34 35 14169

R(K3,K3,K4) 30 31 33 7878R(K3,K3,K5) 45 57 61 8433R(K3,K4,K4) 55 79 85 15625R(K3,K3,K3,K3) 51 62 65 18571R(K4,K

−6 ) 30 33 33 11372

R(K3,C4,K4) 27 32 32 9928R(C4,C4,K4) 20 22 22 11857R(K−

6 1,K−6 ) 45 70 71 9478

Thank you for your attention!

Upper bounds on small Ramsey numbersRamsey NumbersFlag AlgebraApplicationResultsExample Flag...

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