Dana Ernst
April 21, 2012
290

# Classification of the T-avoiding elements in Coxeter groups of type F

Coxeter groups can be thought of as generalized reflections groups. In particular, a Coxeter group is generated by a set of elements of order two. Every element of a Coxeter group can be written as an expression in the generators, and if the number of generators in an expression is minimal, we say that the expression is reduced. We say that an element w of a Coxeter group is T- avoiding if w does not have a reduced expression beginning or ending with a pair of non-commuting generators. In this talk, we will classify the T-avoiding elements of type F_5. We conjecture that our classification holds more generally for F_n with n ≥ 5.

This talk was given by my undergraduate research students Ryan Cross, Katie Hills-Kimball, and Christie Quaranta (Plymouth State University) on April 21, 2012 at the 2012 Hudson River Undergraduate Mathematics Conference at Western New England University.

April 21, 2012

## Transcript

1. ### Classiﬁcation of the T-avoiding elements in Coxeter groups of type

F R. Cross, K. Hills-Kimball, C. Quaranta Directed by D.C. Ernst Plymouth State University Mathematics Department Western New England University April 21, 2012 Cross, Hills-Kimball, Quaranta T-avoiding elements in Coxeter groups of type F 1 / 1
2. ### Coxeter groups Deﬁnition A Coxeter system consists of a group

W (called a Coxeter group) generated by a set S of involutions with presentation W = S : s2 = 1, (st)m(s,t) = 1 , where m(s, t) ≥ 2 for s = t. Since s and t are involutions, the relation (st)m(s,t) = 1 can be rewritten as m(s, t) = 2 =⇒ st = ts short braid relations m(s, t) = 3 =⇒ sts = tst m(s, t) = 4 =⇒ stst = tsts . . .          long braid relations Coxeter groups can be thought of as generalized reﬂection groups. Cross, Hills-Kimball, Quaranta T-avoiding elements in Coxeter groups of type F 2 / 1
3. ### Coxeter graphs Deﬁnition We can encode (W , S) with

a unique Coxeter graph X having: 1. vertex set S; 2. edges {s, t} labeled m(s, t) whenever m(s, t) ≥ 3. Comments • Typically labels of m(s, t) = 3 are omitted. • Edges correspond to non-commuting pairs of generators. • Given X, we can uniquely reconstruct the corresponding (W , S). Cross, Hills-Kimball, Quaranta T-avoiding elements in Coxeter groups of type F 3 / 1
4. ### Type A Example The Coxeter group of type A3 is

deﬁned by the graph below. s1 s2 s3 Figure: Coxeter graph of type A3. Then W (A3 ) is subject to: • s2 i = 1 for all i • s1 s2 s1 = s2 s1 s2 , s2 s3 s2 = s3 s2 s3 • s1 s3 = s3 s1 In this case, W (A3 ) is isomorphic to the symmetric group Sym4 under the correspondence s1 ↔ (1 2), s2 ↔ (2 3), s3 ↔ (3 4). Cross, Hills-Kimball, Quaranta T-avoiding elements in Coxeter groups of type F 4 / 1
5. ### Type B Example The Coxeter group of type B4 is

deﬁned by the graph below. s1 s2 s3 s4 4 Figure: Coxeter graph of type B4. Then W (B4 ) is subject to: • s2 i = 1 for all i • s2 s3 s2 = s3 s2 s3, s3 s4 s3 = s4 s3 s4 • s1 s2 s1 s2 = s2 s1 s2 s1 • Non-connected nodes commute In this case, W (B4 ) is isomorphic to the group that rearranges and ﬂips 3 coins. Cross, Hills-Kimball, Quaranta T-avoiding elements in Coxeter groups of type F 5 / 1
6. ### Type F Example The Coxeter group of type F5 is

deﬁned by the graph below. s1 s2 s3 s4 s5 4 Then W (F5 ) is subject to: • s2 i = 1 for all i • s1 s2 s1 = s2 s1 s2 ; s3 s4 s3 = s4 s3 s4 ; s4 s5 s4 = s5 s4 s5 • s2 s3 s2 s3 = s3 s2 s3 s2 • Non-connected nodes commute F4 is a ﬁnite group, however Fn for n ≥ 5 is an inﬁnite group. Cross, Hills-Kimball, Quaranta T-avoiding elements in Coxeter groups of type F 6 / 1
7. ### Reduced expressions & Matsumoto’s theorem Deﬁnition A word sx1 sx2

· · · sxm ∈ S∗ is called an expression for w ∈ W if it is equal to w when considered as a group element. If m is minimal, it is a reduced expression. Example Consider the expression s1 s3 s2 s1 s2 for an element w ∈ W (A3 ). Note that s1 s3 s2 s1 s2 = s1 s3 s1 s2 s1 = s3 s1 s1 s2 s1 = s3 s2 s1. Therefore, s1 s3 s2 s1 s2 is not reduced. However, the expression on the right is reduced. Theorem (Matsumoto/Tits) Any two reduced expressions for w ∈ W diﬀer by a sequence of braid relations. Cross, Hills-Kimball, Quaranta T-avoiding elements in Coxeter groups of type F 7 / 1
8. ### Heaps One way of representing reduced expressions is via heaps.

Fix a reduced expression sx1 sx2 · · · sxm for w ∈ W (for any straight line Coxeter graph). Loosely speaking, the heap for this expression is a set of lattice points, one for each sxi , embedded in N × N such that: • The node corresponding to sxi has vertical component equal to n + 1 − xi (smaller numbers at the top), • If i < j and sxi does not commute with sxj , then sxi occurs to the left of sxj . Example Consider s1 s2 s3 s2 , s1 s3 s2 s3 , and s3 s1 s2 s3 , which are all reduced expressions of the same element in A3 . It turns out, there are two distinct heaps: 1 2 3 2 and 1 3 2 3 Comment If two reduced expressions diﬀer by a sequence of short braid relations (i.e., commutations), then they have the same heap. Cross, Hills-Kimball, Quaranta T-avoiding elements in Coxeter groups of type F 8 / 1
9. ### Property T and T-avoiding Deﬁnition We say that w ∈

W has Property T iﬀ some reduced expression begins or ends with a product of non-commuting generators. That is, w = s t (other crap) or w = (other crap) t s Deﬁnition We say that w is T-avoiding iﬀ w does not have Property T. Proposition Products of commuting generators are T-avoiding. Question Are there other elements besides products of commuting generators that are T-avoiding? Cross, Hills-Kimball, Quaranta T-avoiding elements in Coxeter groups of type F 9 / 1
10. ### T-avoiding in types A, B, C, and D Deﬁnition An

element is classiﬁed as bad iﬀ it is T-avoiding, but not a product of commuting generators. Theorem (Cormier, Ernst, Goldenberg, Kelly, Malbon) In types A and B, there are no bad elements. In other words, w ∈ W is T-avoiding iﬀ w is a product of commuting generators. Comment The answer isn’t so simple in other Coxeter groups. In particular, there are bad elements in types C (Ernst) and D (Tyson Gern). Cross, Hills-Kimball, Quaranta T-avoiding elements in Coxeter groups of type F 10 / 1
11. ### Bowties, M’s, and Seagulls Proposition (Cross, Ernst, Hills-Kimball, Quaranta) The

following reduced expressions are bad elements in F5 : Bowtie M Seagull 1 3 5 2 4 3 2 4 1 3 5 1 3 5 2 3 4 3 2 1 3 5 1 3 2 3 4 5 4 3 2 1 3 We can convert Bowties ↔ M’s ↔ Seagulls via braid relations. These expressions represent the same group element. We will restrict our attention to the bowties. We can also stack bowties to create inﬁnitely many bad elements in F5 . 1 3 5 2 4 3 2 4 1 3 5 2 4 3 2 4 1 3 5 · · · 1 3 5 2 4 3 2 4 1 3 5 Cross, Hills-Kimball, Quaranta T-avoiding elements in Coxeter groups of type F 11 / 1
12. ### Results Theorem (Cross, Ernst, Hills-Kimball, Quaranta) An element is T-avoiding

in F5 iﬀ it is a product of commuting generators or a stack of bowties. Sketch of Proof (⇐) Easy. (⇒) Hard. Here’s an outline. • Every bad element must begin or end with 135 or 13. • If w is bad, then w begins and ends with a bowtie, M, or seagull. Cross, Hills-Kimball, Quaranta T-avoiding elements in Coxeter groups of type F 12 / 1
13. ### Results Cross, Hills-Kimball, Quaranta T-avoiding elements in Coxeter groups of

type F 13 / 1
14. ### Results Theorem (Cross, Ernst, Hills-Kimball, Quaranta) An element is T-avoiding

in F5 iﬀ it is a product of commuting generators or a stack of bowties. Sketch of Proof (⇐) Easy. (⇒) Hard. Here’s an outline. • Every bad element must begin or end with 135 or 13. • If w is bad, then w begins and ends with a bowtie, M, or seagull. • Let b = 1 3 5 2 4 3 2 4 . If w = bk 1 3 5 t f is bad, then f is bad, as well. → We proved the contrapositive: If f has Property T, then w also has Property T. Cross, Hills-Kimball, Quaranta T-avoiding elements in Coxeter groups of type F 14 / 1
15. ### Results (continued) Corollary (Cross, Ernst, Hills-Kimball, Quaranta) There are no

bad elements in F4 . That is, the only T-avoiding elements in F4 are products of commuting generators. Conjecture An element is T-avoiding in Fn for n ≥ 5 iﬀ it is a product of commuting generators or a stack of bowties times products of commuting generators. In other words, there are no new bad elements in Fn for n ≥ 6. THANK YOU! Cross, Hills-Kimball, Quaranta T-avoiding elements in Coxeter groups of type F 15 / 1