Graph Topology and Discrete Morse Theory - GitHub PagesDiscrete Morse theory was developed by Robin...

Origins Focusing on Graphs Results Future Directions Appendix

Graph Topology and Discrete Morse Theory

Andrew Rosevear, Andrew TawfeekAdvised by Prof. Ivan Contreras

Amherst College

July 23rd, 2018

Outline Part I

OriginsSmooth Morse TheoryDiscrete Morse Theory

Focusing on GraphsDefinitionsGradient Flow

ResultsCharacterizing Critical CellsCharacterizing Gradient FlowWeak Morse Inequalities

Future Directions

Outline Part I

Future Directions

Outline Part I

Future Directions

Outline Part I

Future Directions

Smooth Morse TheoryBuilding Intuition

Note that an important result in smooth Morse theory is that given acritical point, we can choose the correct local coordinates so the functiontakes the form of a parabaloid opening upwards/downwards or a saddlepoint.

Lastly, it turns out that there is an important correspondence betweenMorse functions f and gradient-like vector fields for f .

Shifting View

Discrete Morse theory was developed by Robin Forman around2002, in his published work A Users Guide to Discrete MorseTheory.

Here, he develops an adaption of smooth Morse theory forCW complexes1 that preserves many discrete analogues to theproperties of Morse functions in smooth Morse theory.

We will only focus on the definition with 0-cells (vertices) and1-cells (edges), i.e. graphs.

1CW complexes can be regarded as a generalization of graphs, where notonly can you glue points and edges (S0) together, but higher dimensionalspheres as well.

Shifting View

Discrete Morse theory was developed by Robin Forman around2002, in his published work A Users Guide to Discrete MorseTheory. Here, he develops an adaption of smooth Morse theory forCW complexes1 that preserves many discrete analogues to theproperties of Morse functions in smooth Morse theory.

Shifting View

Discrete Morse theory was developed by Robin Forman around2002, in his published work A Users Guide to Discrete MorseTheory. Here, he develops an adaption of smooth Morse theory forCW complexes1 that preserves many discrete analogues to theproperties of Morse functions in smooth Morse theory.

Focusing on GraphsDefinitions

For a graph Γ, define an ordering on the cells, Γc = V ∪ E , bydeclaring every vertex lesser than the edge of which it is anendpoint.

DefinitionLet Γ = (V ,E ) be a graph. A discrete Morse function is a functionf : Γc = V ∪ E → R such that for every σ ∈ Γc ,

|{τ ∈ Γc | σ < τ and f (σ) ≥ f (τ)}| ≤ 1; (1)

|{τ ∈ Γc | σ > τ and f (σ) ≤ f (τ)}| ≤ 1. (2)

We say a cell (vertex or edge) is critical if both sets (1) and (2)above are empty. We let c0(f ) denote the number of criticalvertices, and c1(f ) number of critical edges.

|{τ ∈ Γc | σ < τ and f (σ) ≥ f (τ)}| ≤ 1; (1)

|{τ ∈ Γc | σ > τ and f (σ) ≤ f (τ)}| ≤ 1. (2)

|{τ ∈ Γc | σ < τ and f (σ) ≥ f (τ)}| ≤ 1; (1)

|{τ ∈ Γc | σ > τ and f (σ) ≤ f (τ)}| ≤ 1. (2)

ResultsGradient Flow

ResultsCharacterizing Critical Cells

We call this oriented graph corresponding to a pair (Γ, f ) of agraph and it’s discrete Morse function the gradient flow Γf .

Theorem (A.T.)

1. edge is critical ⇐⇒ undirected in Γf

2. vertex is a critical ⇐⇒ sink in Γf

Another note to have in mind is critical vertices occur at the endof gradient curves.

In fact, we can do even better. We can fully characterize discreteMorse functions with gradient flows.

Theorem (A.T.)

ResultsCharacterizing Gradient Flow

Theorem (A.T)Let Γo be a directed graph and Γ be it’s underlying undirected graph. ThenΓo = Γf for some discrete Morse function f on Γ if and only if

1. no two edges share a tail, and

2. there are no directed loops.

If we denote by Morse(Γ) be the set of all discrete Morse functions that can bedefined on Γ, then

f ∼Γ g ⇐⇒ Γf∼= Γg

is an equivalence relation on Morse(Γ).

This turns out to be equivalent to Forman’s equivalence of discrete Morsefunctions on a graph, i.e. f is equivalent to g if and only if for every vertex andedge of Γ,

f (v) < f (e) ⇐⇒ g(v) < g(e).

ResultsWeak Morse Inequalities

Recall that the 0th Betti number of a graph, b0(Γ), is number ofconnected components, and the 1st Betti number, b1(Γ), is thenumber of independent cycles.

Additionally, the Eulercharacteristic χ(Γ) can be expressed as b0 − b1 = |V | − |E |.

Theorem (Weak Morse Inequalities, A.T.)

Let Γ be a simple and finite graph and f : Γc → R be a Morsefunction on the graph.Then

• b0(Γ) ≤ c0(f )

• b1(Γ) ≤ c1(f )

• χ(Γ) = c0(f )− c1(f )

Additionally, there have been numerous technical results concerningthe equivalence classes that are still being studied in more depth.

Recall that the 0th Betti number of a graph, b0(Γ), is number ofconnected components, and the 1st Betti number, b1(Γ), is thenumber of independent cycles. Additionally, the Eulercharacteristic χ(Γ) can be expressed as b0 − b1 = |V | − |E |.

• b0(Γ) ≤ c0(f )

• b1(Γ) ≤ c1(f )

• χ(Γ) = c0(f )− c1(f )

Let Γ be a simple and finite graph and f : Γc → R be a Morsefunction on the graph.

• b0(Γ) ≤ c0(f )

• b1(Γ) ≤ c1(f )

• χ(Γ) = c0(f )− c1(f )

• b0(Γ) ≤ c0(f )

• b1(Γ) ≤ c1(f )

• χ(Γ) = c0(f )− c1(f )

• b0(Γ) ≤ c0(f )

• b1(Γ) ≤ c1(f )

• χ(Γ) = c0(f )− c1(f )

• b0(Γ) ≤ c0(f )

• b1(Γ) ≤ c1(f )

• χ(Γ) = c0(f )− c1(f )

• b0(Γ) ≤ c0(f )

• b1(Γ) ≤ c1(f )

• χ(Γ) = c0(f )− c1(f )

Future Directions

• Study the number of equivalence classes for a given graph Γ.

• The number depends on the automorphism group Aut(Γ), andhow it acts on a special set of colored-graphs relating to Γ.

• We know the number when the automorphism group is trivial,and are very close to obtaining it for an arbitrary graph.

• Expand from vertex-gluing to interface-gluing discrete Morsefunctions and their corresponding graphs.

• Study properties of Morse complexes MΓ(f ) and theirrelation to the equivalence classes.

• By observing the barycentric subdivision of simiplicialcomplexes and regular CW complexes, attempt to framegeneral discrete Morse theory in terms of these equivalenceclasses.

• Develop an analogue of discrete Morse theory for hypergraphs.

Future Directions

• Study the number of equivalence classes for a given graph Γ.• The number depends on the automorphism group Aut(Γ), and

how it acts on a special set of colored-graphs relating to Γ.

• We know the number when the automorphism group is trivial,and are very close to obtaining it for an arbitrary graph.

Future Directions

how it acts on a special set of colored-graphs relating to Γ.• We know the number when the automorphism group is trivial,

and are very close to obtaining it for an arbitrary graph.

Future Directions

Bibliography

I. Contreras, B. Xu.The Graph Laplacian and Morse InequalitiesarXiv:1704.08354

K. Knudson.Morse Theory: Smooth and Discrete.World Scientific Publishing Co. Pte. Ltd., 2015.

L. Schneps.Hodge Theory and Complex Algebraic Geometry ICambridge University Press, 2002

R. Forman.A User’s Guide to Discrete Morse Theory.Seminaire Lotharingien de Combinatoire, 2002.

S. Lang.Fundamentals of Differential GeometrySpringer Science Business Media, 2001

Topology of Graphs Graph de Rham Calculus Future Directions

Outline

Topology of GraphsChain ComplexesThe Graph LaplacianGraph Homology

Graph de Rham CalculusDifferentiationIntegrationMain Results

Graph Stokes’ TheoremGraph Hodge Decomposition

Future DirectionsSolving Eigenvector IntegrationThe Morse ComplexAnalyzing Graph DiffEqs

Graph Chain Complexes

Definition (Chain Complex of a Graph)

The chain complex of a directed graph Γ = (V ,E ) is asequence of vector spaces paired with linear maps

0→ C|E | ∂1−→ C|V | → 0,

where ∂1 is the boundary operator given by the |V | × |E |incidence matrix I whose entries are

1 edge ej enters vertex vi

−1 edge ej leaves vertex vi

0 otherwise

The Graph Laplacian

Definition (Graph Laplacian)

The even and odd graph Laplacians ∆+ and ∆− of an orientedgraph Γ are given by

∆+ := II ∗ : C|V | → C|V |

∆− := I ∗I : C|E | → C|E |.

Both matrices are positive semidefinite and symmetric (andtherefore diagonalizable).

Lemma∆+ is invariant under orientation. However, ∆− is not.

The Graph Laplacian

Definition (Graph Laplacian)

The even and odd graph Laplacians ∆+ and ∆− of an orientedgraph Γ are given by

∆+ := II ∗ : C|V | → C|V |

∆− := I ∗I : C|E | → C|E |.

Both matrices are positive semidefinite and symmetric (andtherefore diagonalizable).

Lemma∆+ is invariant under orientation. However, ∆− is not.

Homology and Betti Numbers

Definition (Homology Groups)

The homology groups of a graph Γ are given by

H1(Γ) = ker(I ) = ker(∆−)

andH0(Γ) = ker(I ∗) = ker(∆+).

Theorem (Contreras-Xu)

Let b1 and b0 be the Betti numbers of Γ. Then

dim(H1(Γ)) = b1

dim(H0(Γ)) = b0.

Homology and Betti Numbers

Definition (Homology Groups)

The homology groups of a graph Γ are given by

H1(Γ) = ker(I ) = ker(∆−)

andH0(Γ) = ker(I ∗) = ker(∆+).

Theorem (Contreras-Xu)

Let b1 and b0 be the Betti numbers of Γ. Then

dim(H1(Γ)) = b1

dim(H0(Γ)) = b0.

Cochain Complexes and the Graph Differential

The graph cochain complex is simply the graph chain complexbut with the arrows reversed, and I replaced with I ∗:

0→ C|V | I∗−→ C|E | → 0.

By analogy with the differential operator on the de Rham complex,we may view I ∗ as a kind of graph differential operator.In vector calculus terminology, I ∗ serves as the graph gradient.

Cochain Complexes and the Graph Differential

The graph cochain complex is simply the graph chain complexbut with the arrows reversed, and I replaced with I ∗:

0→ C|V | I∗−→ C|E | → 0.

By analogy with the differential operator on the de Rham complex,we may view I ∗ as a kind of graph differential operator.In vector calculus terminology, I ∗ serves as the graph gradient.

Integration on Graphs

Definition (Vertex Integral)

Let f be a discrete function on the vertices of Γ. The vertexintegral of f over Γ is given by∫ •

∂Γf =

∑vi∈V

f (vi )dn(vi ),

where dn(vi ) is the number of incoming minus outgoing edges.

Definition (Edge Integral)

Let F be a discrete function on the edges of Γ. The edge integralof F over Γ is given by∫ −

∑ei∈E

F (ei ).

Integration on Graphs

Definition (Vertex Integral)

Let f be a discrete function on the vertices of Γ. The vertexintegral of f over Γ is given by∫ •

∂Γf =

∑vi∈V

f (vi )dn(vi ),

where dn(vi ) is the number of incoming minus outgoing edges.

Definition (Edge Integral)

Let F be a discrete function on the edges of Γ. The edge integralof F over Γ is given by∫ −

∑ei∈E

F (ei ).

Main Results

Theorem (A.R)

(Stokes’ Theorem for Graphs) Let f be a vertex function on anoriented graph Γ. Then ∫ •

∂Γf =

∫ −Γ

I ∗f .

Theorem (A.R)

(Graph Hodge Decomposition) Let Γ be an oriented graph andH0(Γ) = ker(∆+) = ker(I ∗) and H1(Γ) = ker(∆−) = ker(I ) its nthhomology group. Then

C|V | = H0(Γ)⊕ Im(I )

C|E | = H1(Γ)⊕ Im(I ∗).

Main Results

Theorem (A.R)

(Stokes’ Theorem for Graphs) Let f be a vertex function on anoriented graph Γ. Then ∫ •

∂Γf =

∫ −Γ

I ∗f .

Theorem (A.R)

(Graph Hodge Decomposition) Let Γ be an oriented graph andH0(Γ) = ker(∆+) = ker(I ∗) and H1(Γ) = ker(∆−) = ker(I ) its nthhomology group. Then

C|V | = H0(Γ)⊕ Im(I )

C|E | = H1(Γ)⊕ Im(I ∗).

Future Directions

1. What happens when we integrate an eigenvector with nonzeroeigenvalue of ∆+ or ∆−?

2. One may consider the Morse complex, the chain complex ofthe subgraph induced by the critical cells of a Morse graph.

2.1 For an alternate proof of the Morse Inequalities, seeContreras-Xu, ”The Graph Laplacian and Morse Inequalities.”

3. A more coherent theory of graph calculus can help answerquestions about graph differential equations, such as thegraph Schrodinger equation

∂t= i∆ϕ.

Future Directions

∂t= i∆ϕ.

Future Directions

∂t= i∆ϕ.

Graph Topology and Discrete Morse Theory - GitHub PagesDiscrete Morse theory was developed by Robin...

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