Introduction
Diagrammatics
Techniques
Oriented Cases
A Diagrammatic Approach to the K(π, 1)
Conjecture
Niket Gowravaram and Uma Roy
Mentor: Alisa Knizel
Fourth Annual MIT PRIMES Conference
May 18, 2014
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Introduction
Diagrammatics
Techniques
W HAT IS A C OXETER GROUP ?
Oriented Cases
Conclusion
Introduction
Diagrammatics
Techniques
Oriented Cases
W HAT IS A C OXETER GROUP ?
Definition
A Coxeter group is given by generators g1 , g2 , . . . , gn with
relations:
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g2i = 1 for all i
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(gi gj )mij = 1 for all i, j
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Introduction
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Oriented Cases
Conclusion
W HAT IS A C OXETER GROUP ?
Definition
A Coxeter group is given by generators g1 , g2 , . . . , gn with
relations:
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g2i = 1 for all i
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(gi gj )mij = 1 for all i, j
Some examples of Coxeter groups include the symmetric group
and reflection groups.
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C OXETER D IAGRAMS
Coxeter diagrams can be used to visualize Coxeter groups.
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Each vertex represents a generator
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Edges show the relations between generators
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D IAGRAMMATICS
We can use Coxeter groups to create certain graphs.
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D IAGRAMMATICS
We can use Coxeter groups to create certain graphs.
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Each color represents a generator.
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The degree of each vertex is determined by the relations
between generators.
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Conclusion
K(π, 1) C ONJECTURE
There is a conjecture known as the K(π, 1) conjecture regarding
the second homotopy group of the dual Coxeter complex.
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The dual Coxeter complex is a topological space associated
to each Coxeter group
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Elements of second homotopy group correspond to
aforementioned graphs
Proving this conjecture is equivalent to proving that all possible
graphs for a Coxeter group can be simplfied to the empty
graph using a sequence of allowed moves.
Introduction
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Techniques
M OVES ON D IAGRAMS
How can we simplify a graph?
Oriented Cases
Conclusion
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M OVES ON D IAGRAMS
How can we simplify a graph?
3 allowable moves:
Oriented Cases
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M OVES ON D IAGRAMS
How can we simplify a graph?
3 allowable moves:
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Circle relation
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Bridge relation
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Zamolodchikov relations
Oriented Cases
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C IRCLE R ELATION
We are allowed to add or remove empty circles of any color.
LALALALALALALAL
=
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Conclusion
B RIDGE R ELATION
If we have two edges of the same color, we can switch around
which vertices they connect to, as long as we do not create any
new intersections.
=
Introduction
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Conclusion
Z AMOLODCHIKOV R ELATIONS (ZAM R ELATIONS )
Zam relations vary for different coxeter groups. They are found
through the reduced expression graph for the longest element.
Introduction
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Conclusion
Z AMOLODCHIKOV R ELATIONS (ZAM R ELATIONS )
Zam relations vary for different coxeter groups. They are found
through the reduced expression graph for the longest element.
A3 :
=
=
=
Introduction
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O UR P ROJECT
In our project, our primary goal was to prove the K(π, 1)
conjecture for specific Coxeter groups.
Conclusion
Introduction
Diagrammatics
Techniques
Oriented Cases
O UR P ROJECT
In our project, our primary goal was to prove the K(π, 1)
conjecture for specific Coxeter groups.
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I2 (m)
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A3
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B3
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G×H
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Directed cases
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Working on An
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A DJACENT V ERTICES
Theorem
We can remove adjacent vertices of the same type.
Conclusion
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Oriented Cases
A DJACENT V ERTICES
Theorem
We can remove adjacent vertices of the same type.
Proof.
→
Conclusion
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Oriented Cases
A DJACENT V ERTICES
Theorem
We can remove adjacent vertices of the same type.
Proof.
→
→
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A DJACENT V ERTICES
Theorem
We can remove adjacent vertices of the same type.
Proof.
→
→
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I2 (m)
I2 (m):
m
Theorem
The family of Coxeter groups I2 (m) satisfies the K(π, 1) conjecture.
Conclusion
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I2 (m)
I2 (m):
m
Theorem
The family of Coxeter groups I2 (m) satisfies the K(π, 1) conjecture.
Proof.
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Only 1 type of vertex so necesarily 2 adjacent vertices of
same type
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I2 (m)
I2 (m):
m
Theorem
The family of Coxeter groups I2 (m) satisfies the K(π, 1) conjecture.
Proof.
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Only 1 type of vertex so necesarily 2 adjacent vertices of
same type
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Use induction on number of vertices
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A3
A3 :
Theorem
The Coxeter group A3 satisfies the K(π, 1) conjecture.
Conclusion
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A3
A3 :
Theorem
The Coxeter group A3 satisfies the K(π, 1) conjecture.
Strategy: Look at subgraph of blue color and use Euler
characteristic: V + F = E + 2 to find a small face.
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Delete the small face.
Conclusion
Introduction
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Techniques
Oriented Cases
A3
A3 :
Theorem
The Coxeter group A3 satisfies the K(π, 1) conjecture.
Strategy: Look at subgraph of blue color and use Euler
characteristic: V + F = E + 2 to find a small face.
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Delete the small face.
Using parity, the only nontrivial case is a blue face with 4
edges.
Conclusion
Introduction
Diagrammatics
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Oriented Cases
A3
Any face with 4 edges can be transformed into ZAM for A3 .
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Look at continuation of edges outside of face
Conclusion
Introduction
Diagrammatics
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Oriented Cases
A3
Any face with 4 edges can be transformed into ZAM for A3 .
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Look at continuation of edges outside of face
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Use bridge relation to connect edges
Conclusion
Introduction
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Oriented Cases
Conclusion
A3
Any face with 4 edges can be transformed into ZAM for A3 .
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Look at continuation of edges outside of face
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Use bridge relation to connect edges
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After using Zam transform, there must be adjacent vertices
of the same type.
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B3
B3 :
Theorem
The Coxeter group B3 satisfies the K(π, 1) conjecture.
Conclusion
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B3
B3 :
Theorem
The Coxeter group B3 satisfies the K(π, 1) conjecture.
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We examine the subgraph of green color.
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B3
B3 :
Theorem
The Coxeter group B3 satisfies the K(π, 1) conjecture.
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We examine the subgraph of green color.
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Use Euler characteristic to find a small green face.
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Conclusion
B3
B3 :
Theorem
The Coxeter group B3 satisfies the K(π, 1) conjecture.
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We examine the subgraph of green color.
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Use Euler characteristic to find a small green face.
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Faces with odd number of edges necesarily have adjacent
vertices of the same type that can be removed. Using
parity and more complicated arguments, faces with 2 or 4
edges also necesarily have vertices that can be removed.
Introduction
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Conclusion
B3
B3 :
Theorem
The Coxeter group B3 satisfies the K(π, 1) conjecture.
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We examine the subgraph of green color.
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Use Euler characteristic to find a small green face.
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Faces with odd number of edges necesarily have adjacent
vertices of the same type that can be removed. Using
parity and more complicated arguments, faces with 2 or 4
edges also necesarily have vertices that can be removed.
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Only nontrivial case is a green face with 6 edges. Vertices
of type green-red and green-blue alternate around face.
Introduction
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B3
Using idea that no adjacent vertices can be of same type, we
can manipulate this face into the B3 ZAM relation.
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Examine continuation of edges outside of face.
Conclusion
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B3
Using idea that no adjacent vertices can be of same type, we
can manipulate this face into the B3 ZAM relation.
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Examine continuation of edges outside of face.
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Only 1 vertex of type red-blue inside face.
Conclusion
Introduction
Diagrammatics
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Oriented Cases
B3
Using idea that no adjacent vertices can be of same type, we
can manipulate this face into the B3 ZAM relation.
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Examine continuation of edges outside of face.
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Only 1 vertex of type red-blue inside face.
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Use bridge relation inside and outside of face to connect
edges.
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Use ZAM transformation and get adjacent vertices of the
same type.
Conclusion
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G×H
Theorem
If the K(π, 1) conjecture holds for groups G and H, then it holds for
the group G × H.
Conclusion
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G×H
Theorem
If the K(π, 1) conjecture holds for groups G and H, then it holds for
the group G × H.
Strategy: Commutative Colors
Conclusion
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Oriented Cases
A1 × H
Theorem
If two generators commute, then we can move the edges
corresponding to them independently.
Conclusion
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Conclusion
A1 × H
Theorem
If two generators commute, then we can move the edges
corresponding to them independently.
Using this idea, we can solve the general case G × H by
essentially separating the graph formed by the generators of G
from the one formed by the generators of H.
Introduction
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Techniques
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O RIENTED G RAPHS
We also solved some cases involving oriented graphs.
Conclusion
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O RIENTED A2
Oriented cases are much more difficult because we cannot
necessarily remove adjacent vertices of the same type.
Conclusion
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Diagrammatics
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Oriented Cases
O RIENTED A2
Oriented cases are much more difficult because we cannot
necessarily remove adjacent vertices of the same type.
Strategy: Look at the longest cycle
Conclusion
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Conclusion
F UTURE D IRECTIONS
We can take this project in multiple directions in the future.
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We could continue proving the K(π, 1) conjecture for other
Coxeter groups.
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We could generalize our proofs to classes of Coxeter
groups. (For example, we have a nearly-finished proof for
An .)
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We could also investigate oriented versions of the cases we
have already solved.
Introduction
Diagrammatics
Techniques
Oriented Cases
A CKNOWLEDGEMENTS
We would like to thank:
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Our mentor Alisa Knizel
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PRIMES for providing us with this opportunity
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Dr. Khovanova for the presentation practice
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Dr. Ben Elias for providing the project
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Our parents for their support
Conclusion