Longest element of a Coxeter group

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In mathematics, the longest element of a Coxeter group is the unique element of maximal length in a finite Coxeter group with respect to the chosen generating set consisting of simple reflections. It is often denoted by w0. See (Humphreys 1992, Section 1.8: Simple transitivity and the longest element, pp. 15–16) and (Davis 2007, Section 4.6, pp. 51–53).

Properties[edit]

  • A Coxeter group has a longest element if and only if it is finite; "only if" is because the size of the group is bounded by the number of words of length less than or equal to the maximum.
  • The longest element of a Coxeter group is the unique maximal element with respect to the Bruhat order.
  • The longest element is an involution (has order 2: w_0^{-1} = w_0), by uniqueness of maximal length (the inverse of an element has the same length as the element).[1]
  • For any w \in W, the length satisfies \ell(w_0w) = \ell(w_0) - \ell(w).[1]
  • A reduced expression for the longest element is not in general unique.
  • In a reduced expression for the longest element, every simple reflection must occur at least once.[1]
  • If the Coxeter group is a finite Weyl group then the length of w0 is the number of the positive roots.
  • The open cell Bw0B in the Bruhat decomposition of a semisimple algebraic group G is dense in Zariski topology; topologically, it is the top dimensional cell of the decomposition, and represents the fundamental class.
  • The longest element is the central element –1 except for A_n (n \geq 2), D_n for n odd, E_6, and I_2(p) for p odd, when it is –1 multiplied by the order 2 automorphism of the Coxeter diagram. [2]

See also[edit]

References[edit]

  1. ^ a b c (Humphreys 1992, p. 16)
  2. ^ (Davis 2007, Remark 13.1.8, p. 259)