Can u help me

Algebra Level 4

Let Perm ( X ) \text{Perm}(X) be the group of permutations on a set X X . Let d Z d\in\mathbb Z and d 2 d\ge 2 . The group W = { g Perm ( [ d , d ] Z ) g ( i ) = g ( i ) for all i [ d , d ] Z } W=\{g\in\text{Perm}([-d,d]\cap\mathbb Z)~|~g(-i)=-g(i)\text{ for all }i\in[-d,d]\cap\mathbb Z\} is called Weyl group. Any element w W w\in W is uniquely determined by its value on { 1 , 2 , , d } \{1,2,\dots,d\} , and we shall denote w = [ w ( 1 ) , w ( 2 ) , , w ( d ) ] w=[w(1),w(2),\dots,w(d)] . It's well known that W W is generated by { s 0 , s 1 , , s d 1 } \{s_0,s_1,\dots,s_{d-1}\} , where s 0 = [ 1 , 2 , , d 1 , d ] , s i = [ 1 , , i 1 , i + 1 , i , i + 2 , , d ] , for i = 1 , , d 1. \begin{aligned} &s_0=[-1,2,\dots,d-1,d],\\ &s_i=[1,\dots,i-1,i+1,i,i+2,\dots,d],\text{ for }i=1,\dots,d-1. \end{aligned} Is the following statement correct?

If w = s i 1 s i 2 s i k W w=s_{i_1}s_{i_2}\dots s_{i_k}\in W with s i k = s 0 s_{i_k}=s_0 and k k as small as possible, then w ( 1 ) < 0 w(1)<0 .

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1 solution

Brian Lie
Dec 31, 2020

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