Graph Theory using Sage - Mathematicscgodsil/pdfs/sage.pdf · IntroductionStudent ProjectsMy...

Introduction Student Projects My Projects

Graph Theory using Sage

Chris Godsil

Seattle, August 2009

Chris Godsil Graph Theory using Sage

Introduction Student Projects My Projects

1 IntroductionBackground

2 Student ProjectsConference GraphsThe Matching Polynomial

3 My ProjectsThe 600-CellWalk-Regular GraphsSpectra of Trees

Introduction Student Projects My Projects Background

Outline

I am keen to use sage as a tool for my research.

I want my grad students to learn to make efficient use of it.I would very much like to use it as a tool in undergraduatecombinatorics courses.

I am keen to use sage as a tool for my research.I want my grad students to learn to make efficient use of it.

I would very much like to use it as a tool in undergraduatecombinatorics courses.

I am keen to use sage as a tool for my research.I want my grad students to learn to make efficient use of it.I would very much like to use it as a tool in undergraduatecombinatorics courses.

Obstacles

The chief problem is access: many students are onlycomfortable with windows.

A second problem is competence. Most of my grad studentsknow essentially nothing about combinatorial computing, andnot much about computing.

Obstacles

The chief problem is access: many students are onlycomfortable with windows.A second problem is competence. Most of my grad studentsknow essentially nothing about combinatorial computing, andnot much about computing.

Solutions?

Time will solve the access problem.

Appropriate documentation will help with the competence.(Note that useful python documentation is lacking too.)

Solutions?

Time will solve the access problem.Appropriate documentation will help with the competence.(Note that useful python documentation is lacking too.)

Introduction Student Projects My Projects Conference Graphs The Matching Polynomial

Outline

A Construction

Start with a vector space V of dimension two over a field ofodd order q. There are q + 1 parallel classes of lines; choose asubset S of these with size (q + 1)/2.

The vertices of our graph are the elements of V ; two vectorsu and v are adjacent if u 6= v and the line spanned by v − ulies in one of the parallel classes in S .Our graph will have q2 vertices and valency (q2 − 1)/2. It is aCayley graph for the additive group of V .

A Construction

Start with a vector space V of dimension two over a field ofodd order q. There are q + 1 parallel classes of lines; choose asubset S of these with size (q + 1)/2.The vertices of our graph are the elements of V ; two vectorsu and v are adjacent if u 6= v and the line spanned by v − ulies in one of the parallel classes in S .

Our graph will have q2 vertices and valency (q2 − 1)/2. It is aCayley graph for the additive group of V .

A Construction

Start with a vector space V of dimension two over a field ofodd order q. There are q + 1 parallel classes of lines; choose asubset S of these with size (q + 1)/2.The vertices of our graph are the elements of V ; two vectorsu and v are adjacent if u 6= v and the line spanned by v − ulies in one of the parallel classes in S .Our graph will have q2 vertices and valency (q2 − 1)/2. It is aCayley graph for the additive group of V .

Isomorphism

A number of specializations of these procedure produceinteresting infinite families of graphs.

The problem is two determine when the graphs in distinctfamilies are not isomorphic.

Isomorphism

A number of specializations of these procedure produceinteresting infinite families of graphs.The problem is two determine when the graphs in distinctfamilies are not isomorphic.

Cliques

To this end, it is natural to investigate the clique structure.How large can a clique be?

How many cliques of maximum size on a given vertex?On a given edge?

Cliques

To this end, it is natural to investigate the clique structure.How large can a clique be?How many cliques of maximum size on a given vertex?

On a given edge?

Cliques

To this end, it is natural to investigate the clique structure.How large can a clique be?How many cliques of maximum size on a given vertex?On a given edge?

Our Experience

Sage was definitely the right tool to generate the graphs wewanted.

The networkx clique routine was too slow; my student movedto using cliquer.

Our Experience

Sage was definitely the right tool to generate the graphs wewanted.The networkx clique routine was too slow; my student movedto using cliquer.

Outline

Definitions

DefinitionA matching in a graph is a set of disjoint edges. We denote thenumber of matchings in X of size r by p(X , r). The matchingpolynomial µ(X , t) is given by

µ(X , t) :=∑r≥0

(−1)n−rp(X , r)tn−2r

where n = |V (X)|.

For a forest, the matching polynomial coincides with thecharacteristic polynomial (of the adjacency matrix).

Definitions

DefinitionA matching in a graph is a set of disjoint edges. We denote thenumber of matchings in X of size r by p(X , r). The matchingpolynomial µ(X , t) is given by

µ(X , t) :=∑r≥0

(−1)n−rp(X , r)tn−2r

where n = |V (X)|.

For a forest, the matching polynomial coincides with thecharacteristic polynomial (of the adjacency matrix).

The Tasks

Compute the matching polynomial for all graphs on up to 9vertices (say).

Compute the matching polynomial for individual graphs on up20 vertices.

The Tasks

Compute the matching polynomial for all graphs on up to 9vertices (say).Compute the matching polynomial for individual graphs on up20 vertices.

The Difficulties

All known algorithms for computing this polynomial takeexponential time.

There are 274668 graphs on 9 vertices.My student used polynomials as his data structure, and hencegot bogged down in maxima.

The Difficulties

All known algorithms for computing this polynomial takeexponential time.There are 274668 graphs on 9 vertices.

My student used polynomials as his data structure, and hencegot bogged down in maxima.

The Difficulties

All known algorithms for computing this polynomial takeexponential time.There are 274668 graphs on 9 vertices.My student used polynomials as his data structure, and hencegot bogged down in maxima.

Introduction Student Projects My Projects The 600-Cell Walk-Regular Graphs Spectra of Trees

Outline

120 Vectors in R4

Define f := (1 +√

5)/2 and take all even permutations of allpossible signings of the vector (1, f , 1− f , 0)

To this 96 vectors add the 16 vectors 12(±1,±1,±1,±1).

Complete the set to one of size 120 by adding each of the twosignings of the four standard basis vectors.

120 Vectors in R4

Define f := (1 +√

5)/2 and take all even permutations of allpossible signings of the vector (1, f , 1− f , 0)To this 96 vectors add the 16 vectors 1

2(±1,±1,±1,±1).

Complete the set to one of size 120 by adding each of the twosignings of the four standard basis vectors.

120 Vectors in R4

Define f := (1 +√

5)/2 and take all even permutations of allpossible signings of the vector (1, f , 1− f , 0)To this 96 vectors add the 16 vectors 1

2(±1,±1,±1,±1).Complete the set to one of size 120 by adding each of the twosignings of the four standard basis vectors.

A Graph

For us, the 600-cell is the graph with these 120 vectors as itsvertices, where two vectors are adjacent if they lie at distance 1/f .Equivalently it is the 1-skeleton of the convex polytope generatedby these vectors.

Experience

Our 120 vectors correspond to a subgroup of the unitquaternions. This provided a reasonably convenient way toconstruct the graph.

I was able to settle all the questions I had about this graph,by computation and using the insight gained.Do not use a list comprehension with index i if you want i tocontinue to denote a quaternion.

Experience

Our 120 vectors correspond to a subgroup of the unitquaternions. This provided a reasonably convenient way toconstruct the graph.I was able to settle all the questions I had about this graph,by computation and using the insight gained.

Do not use a list comprehension with index i if you want i tocontinue to denote a quaternion.

Experience

Our 120 vectors correspond to a subgroup of the unitquaternions. This provided a reasonably convenient way toconstruct the graph.I was able to settle all the questions I had about this graph,by computation and using the insight gained.Do not use a list comprehension with index i if you want i tocontinue to denote a quaternion.

Outline

Definitions

DefinitionSuppose X is a graph with adjacency matrix A. We say X is walkregular if the diagonal entries of Ar are all equal, for anynon-negative r .

Examples

Vertex-transitive graphs.

Strongly regular (and distance-regular) graphs.Products of walk-regular graphs: Cartesian, direct, strong.

Examples

Vertex-transitive graphs.Strongly regular (and distance-regular) graphs.

Products of walk-regular graphs: Cartesian, direct, strong.

Examples

Vertex-transitive graphs.Strongly regular (and distance-regular) graphs.Products of walk-regular graphs: Cartesian, direct, strong.

Finding Regular Graphs

There are two sources:Markus Meringer has a program genreg that generatesregular graphs.

McKay’s geng can also be used. It is less efficient for largergraphs (v ≥ 15 say).

Finding Regular Graphs

There are two sources:Markus Meringer has a program genreg that generatesregular graphs.McKay’s geng can also be used. It is less efficient for largergraphs (v ≥ 15 say).

Conclusions

The smallest walk-regular graph that is not vertex transitive ison 12 vertices.

Further progress will have to be made without computationalhelp—there are too many graphs.

Conclusions

The smallest walk-regular graph that is not vertex transitive ison 12 vertices.Further progress will have to be made without computationalhelp—there are too many graphs.

Outline

Cospectral Trees

Indistinguishability

The trees in this pair cannot be distinguished by the characteristicpolynomials of:

The trees themselves.

Their complements.Their Laplacians.Their line graphs.Their distance matrices.

The trees themselves.Their complements.

Their Laplacians.Their line graphs.Their distance matrices.

The trees themselves.Their complements.Their Laplacians.

Their line graphs.Their distance matrices.

The trees themselves.Their complements.Their Laplacians.Their line graphs.

Their distance matrices.

The trees themselves.Their complements.Their Laplacians.Their line graphs.Their distance matrices.

More Polynomials

We consider a two variable polynomial. Let D denote the diagonalmatrix of degrees of a graph and consider the polynomial:

det(sI − tA + D).

Graph Theory using Sage - Mathematicscgodsil/pdfs/sage.pdf · IntroductionStudent ProjectsMy...

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