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Thread: Character/ Representation Theory

  1. #1
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    Character/ Representation Theory

    Can anybody help me with these questions as I am completely lost and haven't done any character theory or rep theory for a long time


    (1) Let X be the matrix comprising the character table of a group.
    Show that X is non-singular & hence prove that the number of real-valued characters is equal to the number
    of self-inverse conjugacy classes.

    (2) Show that in a group G of odd order, no element other than the identity
    is conjugate to its inverse. Deduce that in a group of odd order, the trivial character is the only
    irreducible character that is real-valued.
    Last edited by Ackbeet; Mar 14th 2011 at 04:43 AM. Reason: Split threads so no thread has more than two problems.
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  2. #2
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    Quote Originally Posted by Turloughmack View Post
    Can anybody help me with these questions as I am completely lost and haven't done any character theory or rep theory for a long time


    (1) Let X be the matrix comprising the character table of a group.
    Show that X is non-singular & hence prove that the number of real-valued characters is equal to the number
    of self-inverse conjugacy classes.
    This is a sketch of the proof borrowed from the book "Representations and characters of groups" by Liebeck (p264).

    The irreducible character of G are linearly indepdent over $\displaystyle \mathbb{C}$ as functions from G to $\displaystyle \mathbb{C}$, which follows that X is non-singular.

    Let $\displaystyle \bar{X}$ be the complex conjugate of matrix X. For each irreducible character $\displaystyle \chi^i$, it is known that $\displaystyle \bar{\chi^i}$ is also an irreducible character. Thus, we can construct a permutation matrix P induced from $\displaystyle \chi^i \rightarrow \bar{\chi^i}$ such that $\displaystyle PX=\bar{X}$.

    Similarly, for each conjugacy class $\displaystyle C_i(g_i)$, the entries in the column of X corresponding to $\displaystyle C_i(g_i)$ are the complex conjugates of the entries in the column of X corresponding to $\displaystyle \bar{C_i}(g_i^{-1})$, where $\displaystyle C_i$ denotes the conjugacy class of its representative $\displaystyle g_i$, and $\displaystyle \bar{C_i}$ the conjugacy class of $\displaystyle g_i^{-1}$. Similarly, we can construct a permutation matrix Q induced from $\displaystyle C_i \rightarrow \bar{C_i}$ such that $\displaystyle XQ=\bar{X}$. It follows that $\displaystyle P=XQX^{-1}$. Thus, we see that P and Q have the same trace. Recall that trace of a permutation matrix corresponds to the number of fixed points by the permutation. Therefore, the number of irreducible real-valued characters (corresponding to the fixed points of the first permutation) is the trace of P, and the number of self-inverse conjugacy classes (corresponding the the fixed points of the second permutation) is the trace of Q, which are equal.


    (2) Show that in a group G of odd order, no element other than the identity
    is conjugate to its inverse. Deduce that in a group of odd order, the trivial character is the only
    irreducible character that is real-valued.
    Note that $\displaystyle C_i=\bar{C_i}$ iff each $\displaystyle g_i$ is conjugate to $\displaystyle g_i^{-1}$, i.e., $\displaystyle hg_ih^{-1}=g_i^{-1}$ for some $\displaystyle h \in G$. Then, $\displaystyle h^2g_ih^{-2}=g_i$, $\displaystyle h^3g_ih^{-3}=g_i^{-1}$, etc. Since G is of odd order, we see that $\displaystyle g_i=g_i^{-1}$(verify this). Thus, only the identity is conjugate to its inverse. Hence, the self-inverse conjugacy class is {1}. It follows that the number of irreducible real-valued characters is only one by (1).
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  3. #3
    MHF Contributor Drexel28's Avatar
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    Alternatively, one can appeal to the amazing theorem that for a finite group $\displaystyle G$ if $\displaystyle \sqrt{g}=\#\left\{h\in G:h^2=g\right\}$ then the number of self-conjugate representations of $\displaystyle G$ is $\displaystyle \displaystyle \frac{1}{|G|}\sum_{g\in G}\sqrt{g}$. Although, the proof of this comes startling close to the ideas that the previous paster mentioned.
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