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If $A$ is an involutory matrix, i.e. $A^2=I$, then is $A$ diagonalizable?

Godisemo
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    Using Representation theory one can show that every complex matrix of finite order is diagonalisable. –  Jan 17 '12 at 17:38
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    @Benjamin: Or just using the same argument: if $A^n=I$, then $A$ satisfies $t^n-1$, which has $n$ distinct roots over $\mathbb{C}$; hence the minimal polynomial of $A$ over $\mathbb{C}$ has no repeated roots, and hence $A$ is diagonalizable. – Arturo Magidin Jan 17 '12 at 17:46
  • @ArturoMagidin You said it slicker than me. I was thinking Maschke's Theorem. –  Jan 17 '12 at 17:46

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Since $A^2=I$, then $A$ satisfies the polynomial $t^2-1 = (t-1)(t+1)$. Hence, the minimal polynomial of $A$ divides $(t-1)(t+1)$; so the minimal polynomial of $A$ splits and has distinct roots, so $A$ is diagonalizable.

As N.S. points out in the comments, the above fails if you are working in characteristic 2. There, the matrix $$A=\left(\begin{array}{cc} 0 & 1\\ 1 & 0 \end{array}\right)$$ has minimal and characteristic polynomials $t^2+1 = (t+1)^2$, and it is not diagonalizable (the eigenspace of $1$ has dimension $1$). But if $1\neq -1$, you are set.

Arturo Magidin
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