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# Visualizing diagram factorizations in Temperley–Lieb algebras

The Temperley-Lieb Algebra of type A, invented by Temperley and Lieb in 1971, is a finite dimensional associative algebra that arose in the context of statistical mechanics. Later in 1971, R. Penrose showed that this algebra can be realized in terms of certain diagrams. Then in 1987, V. Jones showed that this algebra occurs naturally as a quotient of an algebra whose underlying structure is the symmetric group. Similarly, there is also a diagrammatic representation for the Temperley-Lieb algebra of type B involving decorated diagrams. Multiplying diagrams is easy to do. However, taking a given diagram and finding the corresponding reduced factorization is generally difficult. We have an efficient (and colorful) algorithm for obtaining a reduced factorization for Temperley-Lieb diagrams of types A and B.

This talk was given by my undergraduate research student Sarah Salmon (Northern Arizona University) on April 12, 2014 at the 2014 Arizona–Nevada Academy of Science Annual Meeting. This joint work with Michael Hastings. April 12, 2014

## Transcript

1. ### Visualizing diagram factorizations in Temperley–Lieb algebras Sarah Salmon Northern Arizona

University Department of Mathematics and Statistics [email protected] Arizona–Nevada Academy of Science Annual Meeting April 12, 2014 Directed by Dr. Dana C. Ernst Joint work with Michael Hastings S. Salmon Visualizing diagram factorizations in TL algebras 1 / 20
2. ### Type A Temperley–Lieb diagrams A diagram of type An must

satisfy the following requirements: • The diagram starts with a box with n + 1 nodes along the north face and n + 1 nodes along the south face. • Every node must be connected to exactly one other node by a single edge. • The edges cannot cross. • The edges cannot leave the box. Example Here is a 6-diagram. This is not a diagram. S. Salmon Visualizing diagram factorizations in TL algebras 2 / 20
3. ### The type A Temperley–Lieb diagram algebra TL(An ) is the

Z[δ]-algebra having (n + 1)-diagrams as a basis. When multiplying diagrams, it is possible to obtain a loop. In this case, we replace each loop with a coeﬃcient δ. = δ2 S. Salmon Visualizing diagram factorizations in TL algebras 3 / 20
4. ### Historical context Comments • TL(An ) was discovered in 1971

by Temperley and Lieb as an algebra with abstract generators and a presentation with the relations above. • It ﬁrst arose in the context of integrable Potts models in statistical mechanics. • As well as having applications in physics, TL(An ) appears in the framework of knot theory, braid groups, Coxeter groups and their corresponding Hecke algebras, and subfactors of von Neumann algebras. • Penrose/Kauﬀman used a diagram algebra to model TL(An ) in 1971. • In 1987, Vaughan Jones (awarded Fields Medal in 1990) recognized that TL(An ) is isomorphic to a particular quotient of the Hecke algebra of type An (the Coxeter group of type An is the symmetric group, Sn+1 ). S. Salmon Visualizing diagram factorizations in TL algebras 4 / 20
5. ### Type A diagrams Example For TL(A3 ), we have the

following set of 4-diagrams as a basis: S. Salmon Visualizing diagram factorizations in TL algebras 5 / 20
6. ### Type A diagrams Example For TL(A3 ), we have the

following set of 4-diagrams as a basis: S. Salmon Visualizing diagram factorizations in TL algebras 5 / 20
7. ### Type A generators and relations For TL(An ), we deﬁne

n simple diagrams as follows: di = 1 n + 1 · · · · · · i i + 1 S. Salmon Visualizing diagram factorizations in TL algebras 6 / 20
8. ### Type A generators and relations For TL(An ), we deﬁne

n simple diagrams as follows: di = 1 n + 1 · · · · · · i i + 1 Theorem TL(An ) satisﬁes the following: S. Salmon Visualizing diagram factorizations in TL algebras 6 / 20
9. ### Type A generators and relations For TL(An ), we deﬁne

n simple diagrams as follows: di = 1 n + 1 · · · · · · i i + 1 Theorem TL(An ) satisﬁes the following: • d2 i = δdi ; • di dj = dj di when |i − j| > 1; • di dj di = di when |i − j| = 1. S. Salmon Visualizing diagram factorizations in TL algebras 6 / 20
10. ### Type A generators and relations For TL(An ), we deﬁne

n simple diagrams as follows: di = 1 n + 1 · · · · · · i i + 1 Theorem TL(An ) satisﬁes the following: • d2 i = δdi ; • di dj = dj di when |i − j| > 1; • di dj di = di when |i − j| = 1. Theorem The set of simple diagrams generate all diagrams in the Temperley–Lieb algebra of type An . S. Salmon Visualizing diagram factorizations in TL algebras 6 / 20
11. ### Products of simple diagrams Example Consider the product d1 d2

d1 d3 d2 d4 d3 in TL(A4 ). = = Note that d1 d2 d1 d3 d2 d4 d3 = d1 d3 d2 d4 d3. S. Salmon Visualizing diagram factorizations in TL algebras 7 / 20
12. ### Products of simple diagrams Example Consider the product d1 d2

d1 d3 d2 d4 d3 in TL(A4 ). = = Note that d1 d2 d1 d3 d2 d4 d3 = d1 d3 d2 d4 d3. S. Salmon Visualizing diagram factorizations in TL algebras 7 / 20
13. ### Factorization of type A We have discovered an algorithm to

determine the factorization given a diagram. S. Salmon Visualizing diagram factorizations in TL algebras 8 / 20
14. ### Factorization of type A We have discovered an algorithm to

determine the factorization given a diagram. ←→ S. Salmon Visualizing diagram factorizations in TL algebras 8 / 20
15. ### Factorization of type A We have discovered an algorithm to

determine the factorization given a diagram. ←→ S. Salmon Visualizing diagram factorizations in TL algebras 8 / 20
16. ### Factorization of type A We have discovered an algorithm to

determine the factorization given a diagram. ←→ S. Salmon Visualizing diagram factorizations in TL algebras 8 / 20
17. ### Factorization of type A We have discovered an algorithm to

determine the factorization given a diagram. ←→ S. Salmon Visualizing diagram factorizations in TL algebras 8 / 20
18. ### Factorization of type A We have discovered an algorithm to

determine the factorization given a diagram. ←→ S. Salmon Visualizing diagram factorizations in TL algebras 8 / 20
19. ### Factorization of type A We have discovered an algorithm to

determine the factorization given a diagram. ←→ By our algorithm, the diagram equals d4 d7 d3 d5 d8 d2 d6 d1 d7 . S. Salmon Visualizing diagram factorizations in TL algebras 8 / 20
20. ### Factorization of type A Let’s verify our calculation. d4 d7

d3 d5 d8 d2 d6 d1 d7 = = S. Salmon Visualizing diagram factorizations in TL algebras 9 / 20
21. ### Type B Temperley–Lieb diagrams A diagram must satisfy the restrictions

for type A and the following: • All decorations must be exposed to the west face; • There are a few technical restrictions on what decorations can occur where; • All loops (decorated or not) are replaced with a coeﬃcient, δ. S. Salmon Visualizing diagram factorizations in TL algebras 10 / 20
22. ### Type B Temperley–Lieb diagrams In TL(Bn ), we multiply diagrams

as in type A subject to the following relations: = = = 2 = 2 = 2 S. Salmon Visualizing diagram factorizations in TL algebras 11 / 20
23. ### Type B diagrams Example For TL(B3 ), we have the

following set of 4-diagrams as a basis: S. Salmon Visualizing diagram factorizations in TL algebras 12 / 20
24. ### Type B diagrams Example For TL(B3 ), we have the

following set of 4-diagrams as a basis: S. Salmon Visualizing diagram factorizations in TL algebras 12 / 20
25. ### Type B generators and relations For TL(Bn ), we deﬁne

n simple diagrams as follows (i ≥ 2): d1 = · · · 1 2 n n + 1 , di = 1 n + 1 · · · · · · i i + 1 S. Salmon Visualizing diagram factorizations in TL algebras 13 / 20
26. ### Type B generators and relations For TL(Bn ), we deﬁne

n simple diagrams as follows (i ≥ 2): d1 = · · · 1 2 n n + 1 , di = 1 n + 1 · · · · · · i i + 1 Theorem TL(Bn ) satisﬁes the following: S. Salmon Visualizing diagram factorizations in TL algebras 13 / 20
27. ### Type B generators and relations For TL(Bn ), we deﬁne

n simple diagrams as follows (i ≥ 2): d1 = · · · 1 2 n n + 1 , di = 1 n + 1 · · · · · · i i + 1 Theorem TL(Bn ) satisﬁes the following: • d2 i = δdi ; • di dj = dj di when |i − j| > 1; • di dj di = di when |i − j| = 1 and i, j = 1; S. Salmon Visualizing diagram factorizations in TL algebras 13 / 20
28. ### Type B generators and relations For TL(Bn ), we deﬁne

n simple diagrams as follows (i ≥ 2): d1 = · · · 1 2 n n + 1 , di = 1 n + 1 · · · · · · i i + 1 Theorem TL(Bn ) satisﬁes the following: • d2 i = δdi ; • di dj = dj di when |i − j| > 1; • di dj di = di when |i − j| = 1 and i, j = 1; • di dj di dj = 2di dj if {i, j} = {1, 2}. S. Salmon Visualizing diagram factorizations in TL algebras 13 / 20
29. ### Type B generators and relations For TL(Bn ), we deﬁne

n simple diagrams as follows (i ≥ 2): d1 = · · · 1 2 n n + 1 , di = 1 n + 1 · · · · · · i i + 1 Theorem TL(Bn ) satisﬁes the following: • d2 i = δdi ; • di dj = dj di when |i − j| > 1; • di dj di = di when |i − j| = 1 and i, j = 1; • di dj di dj = 2di dj if {i, j} = {1, 2}. Theorem The set of simple diagrams generate all diagrams in the Temperley–Lieb algebra of type Bn . S. Salmon Visualizing diagram factorizations in TL algebras 13 / 20
30. ### Proof of last relation Proof For i = 1 and

j = 2, d1 d2 d1 d2 = · · · · · · · · · · · · S. Salmon Visualizing diagram factorizations in TL algebras 14 / 20
31. ### Proof of last relation Proof For i = 1 and

j = 2, d1 d2 d1 d2 = · · · · · · · · · · · · = 2 · · · S. Salmon Visualizing diagram factorizations in TL algebras 14 / 20
32. ### Proof of last relation Proof For i = 1 and

j = 2, d1 d2 d1 d2 = · · · · · · · · · · · · = 2 · · · = 2 · · · · · · S. Salmon Visualizing diagram factorizations in TL algebras 14 / 20
33. ### Proof of last relation Proof For i = 1 and

j = 2, d1 d2 d1 d2 = · · · · · · · · · · · · = 2 · · · = 2 · · · · · · = 2d1 d2 S. Salmon Visualizing diagram factorizations in TL algebras 14 / 20
34. ### Product of simple diagrams in type B Example Here is

an example of a product of several simple diagrams in TL(B4 ). d1 d2 d4 d1 d3 d2 = = S. Salmon Visualizing diagram factorizations in TL algebras 15 / 20
35. ### Factorization of type B Example Using a diagram, let’s work

towards the factorization. S. Salmon Visualizing diagram factorizations in TL algebras 16 / 20
36. ### Factorization of type B Example Using a diagram, let’s work

towards the factorization. ←→ S. Salmon Visualizing diagram factorizations in TL algebras 16 / 20
37. ### Factorization of type B Example Using a diagram, let’s work

towards the factorization. ←→ S. Salmon Visualizing diagram factorizations in TL algebras 16 / 20
38. ### Factorization of type B Example Using a diagram, let’s work

towards the factorization. ←→ S. Salmon Visualizing diagram factorizations in TL algebras 16 / 20
39. ### Factorization of type B Example Using a diagram, let’s work

towards the factorization. ←→ S. Salmon Visualizing diagram factorizations in TL algebras 16 / 20
40. ### Factorization of type B Example Using a diagram, let’s work

towards the factorization. ←→ S. Salmon Visualizing diagram factorizations in TL algebras 16 / 20
41. ### Factorization of type B Example Using a diagram, let’s work

towards the factorization. ←→ S. Salmon Visualizing diagram factorizations in TL algebras 16 / 20
42. ### Factorization of type B Example Using a diagram, let’s work

towards the factorization. ←→ S. Salmon Visualizing diagram factorizations in TL algebras 16 / 20
43. ### Factorization of type B Example Using a diagram, let’s work

towards the factorization. ←→ S. Salmon Visualizing diagram factorizations in TL algebras 16 / 20
44. ### Factorization of type B Example Using a diagram, let’s work

towards the factorization. ←→ S. Salmon Visualizing diagram factorizations in TL algebras 16 / 20
45. ### Factorization of type B Example Using a diagram, let’s work

towards the factorization. ←→ Therefore, the original diagram equals d1 d4 d8 d10 d3 d5 d9 d2 d4 d6 d1 d3 d5 d7 d2 d4 d6 d8 d1 d3 d5 d2 d4 . S. Salmon Visualizing diagram factorizations in TL algebras 16 / 20
46. ### Factorization of type B Let’s check our calculation: d1 d4

d8 d10 d3 d5 d9 d2 d4 d6 d1 d3 d5 d7 d2 d4 d6 d8 d1 d3 d5 d2 d4 = = S. Salmon Visualizing diagram factorizations in TL algebras 17 / 20
47. ### An exception Example There is one case where we must

slightly adjust our algorithm. S. Salmon Visualizing diagram factorizations in TL algebras 18 / 20
48. ### An exception Example There is one case where we must

slightly adjust our algorithm. ←→ S. Salmon Visualizing diagram factorizations in TL algebras 18 / 20
49. ### An exception Example There is one case where we must

slightly adjust our algorithm. ←→ S. Salmon Visualizing diagram factorizations in TL algebras 18 / 20
50. ### An exception Example There is one case where we must

slightly adjust our algorithm. ←→ S. Salmon Visualizing diagram factorizations in TL algebras 18 / 20
51. ### An exception Example There is one case where we must

slightly adjust our algorithm. ←→ S. Salmon Visualizing diagram factorizations in TL algebras 18 / 20
52. ### An exception Example There is one case where we must

slightly adjust our algorithm. ←→ S. Salmon Visualizing diagram factorizations in TL algebras 18 / 20
53. ### An exception Example There is one case where we must

slightly adjust our algorithm. ←→ S. Salmon Visualizing diagram factorizations in TL algebras 18 / 20
54. ### An exception Example There is one case where we must

slightly adjust our algorithm. ←→ S. Salmon Visualizing diagram factorizations in TL algebras 18 / 20
55. ### An exception Example There is one case where we must

slightly adjust our algorithm. ←→ S. Salmon Visualizing diagram factorizations in TL algebras 18 / 20
56. ### An exception Example There is one case where we must

slightly adjust our algorithm. ←→ S. Salmon Visualizing diagram factorizations in TL algebras 18 / 20
57. ### An exception Example There is one case where we must

slightly adjust our algorithm. ←→ This diagram equals d5 d4 d6 d3 d2 d1 d2 d3 S. Salmon Visualizing diagram factorizations in TL algebras 18 / 20
58. ### An exception Here is the product of simple diagrams: d5

d4 d6 d3 d2 d1 d2 d3 = = S. Salmon Visualizing diagram factorizations in TL algebras 19 / 20
59. ### An exception Here is the product of simple diagrams: d5

d4 d6 d3 d2 d1 d2 d3 = = S. Salmon Visualizing diagram factorizations in TL algebras 19 / 20
60. ### Further work and acknowledgements Further work Will our algorithm work

on other types where diagrammatic representations are known to exist? For example, TL(Cn ): Acknowledgements • Northern Arizona University • Department of Mathematics and Statistics • Dr. Dana Ernst S. Salmon Visualizing diagram factorizations in TL algebras 20 / 20