Results 1 to 1 of 1

Thread: Linear Algebra Preliminaries in "Finite Reflection Groups" by Grove and Benson

  1. #1
    Super Member Bernhard's Avatar
    Joined
    Jan 2010
    From
    Hobart, Tasmania, Australia
    Posts
    594
    Thanks
    2

    Linear Algebra Preliminaries in "Finite Reflection Groups" by Grove and Benson

    I think, that I have understood the folowing argument in Grove & Benson. Can someone please check my argument?

    Basically, to repeat the text I am trying to understand: (see attachement for Page 1 of Grove and Benson)In the Preliminaries to Grove and Benson "Finite Reflection Groups"

    On page 1 (see attachment) we find the following:

    "If $\displaystyle \{ x_1 , x_2, ... x_n \} $ is a basis for V, let $\displaystyle V_i $ be the subspace spanned by $\displaystyle \{ x_1, ... , x_{i-1} , x_{i+1}, ... x_n \} $, excluding $\displaystyle x_i $.

    If $\displaystyle 0 \ne y_i \in {V_i}^{\perp} $, then $\displaystyle < x_j , y_i > \ = 0 $ for all $\displaystyle j \ne i $, but $\displaystyle < x_i , y_i > \ne 0 $ , for otherwise $\displaystyle y_i \in V^{\perp} = 0 $"

    ================================================== ===================

    Thus given "$\displaystyle \{ x_1 , x_2, ... x_n \} $ is a basis for V, let $\displaystyle V_i $ be the subspace spanned by $\displaystyle \{ x_1, ... , x_{i-1} , x_{i+1}, ... x_n \} $, excluding $\displaystyle x_i $"

    I am trying to show that:"If $\displaystyle 0 \ne y_i \in {V_i}^{\perp} $, then $\displaystyle < x_j , y_i > \ = 0 $ for all $\displaystyle j \ne i $, but $\displaystyle < x_i , y_i > \ne 0 $ , for otherwise $\displaystyle y_i \in V^{\perp} = 0 $"

    ================================================== ===================

    First show the following:

    If $\displaystyle < x_j , y_i > $ = 0 for all j then $\displaystyle y_i $ must equal zero ........ (1)

    -------------------------------------------------------------------------------------------------------------------------------------------

    Proof of (1)

    $\displaystyle y_i $ must belong to V [since it belongs to a subspace $\displaystyle {V_i}^{\perp} $ of V]

    Thus we can express $\displaystyle y_i $ as follows:[

    tex] y_i = a_1 , x_1 + a_2 , x_2 + .... + a_n , x_n [/tex]

    = $\displaystyle < y_i , x_1 > x_1 + < y_i , x_2 > x_2 + ..... + < y_i , x_n > x_n $ .... (2)

    = $\displaystyle < x_1, y_i > x_1 + < x_2 , y_i > x_2 + .... + < x_n , y_i > x_n $ .... (3)

    If every inner product in (2) or (3) is zero then clearly [itex] y_i [/itex] = 0

    {Problem! Expansion (2) or (3) requires $\displaystyle \{ x_1, x_2, .... , x_n \} $ to be orthonormal! But of course it could be made orthonormal!}

    -----------------------------------------------------------------------------------------

    But we know (1) does not hold because we have assumed $\displaystyle y_i \ne \ 0 $

    But we also know that $\displaystyle < x_j , y_i > $ = 0 for all $\displaystyle j \ne i $ since the $\displaystyle x_j $ with $\displaystyle j \ne i $ belong to $\displaystyle V_i $ which is orthogonal to $\displaystyle {V_i}^{\perp} $.Thus $\displaystyle < x_i , y_i > \ne \ 0$, for otherwise $\displaystyle y_i \in {V_i}^{\perp} = \ 0$

    Is this reasoning correct?

    I would appreciate it very much if someone can confirm the correctness of my reasoning.[Problem: Grove and Benson actually conclude the above argument by saying the following: (see attachement)

    But $\displaystyle < x_i , y_i > \ne \ 0$, for otherwise $\displaystyle y_i \in {V}^{\perp} = \ 0$ but I think this is a typo as they mean $\displaystyle y_i \in {V_i}^{\perp} = \ 0$?? Am I correct? ]

    Hope someone can help.

    Peter
    Last edited by Bernhard; Mar 4th 2012 at 12:56 AM.
    Follow Math Help Forum on Facebook and Google+

Similar Math Help Forum Discussions

  1. Replies: 2
    Last Post: Apr 24th 2011, 07:01 AM
  2. Replies: 1
    Last Post: Oct 25th 2010, 04:45 AM
  3. Replies: 1
    Last Post: Jun 4th 2010, 10:26 PM
  4. Linear Algebra "show that" problem
    Posted in the Advanced Algebra Forum
    Replies: 3
    Last Post: Feb 6th 2009, 02:47 PM
  5. Replies: 1
    Last Post: Nov 2nd 2008, 05:12 PM

Search Tags


/mathhelpforum @mathhelpforum