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  1. Can you help find a $2\times 2$ matrix with eigenvalues $1,-1$ that is not a normal matrix?

    I really tried to find one but the matrix I found is also normal!!

  2. $A$ hermitian and $B$ unitary matrix and AB=BA I need to show $AB$ is normal matrix.

    Well I know that both $A,B$ are normal and that $A=A^*$ and $B=B^{-1}$
    Then $(AB)(AB)^* = (AB)(A^*B^*)$ but now Im not sure what is ok to do for showing $AB$ is normal...

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$(AB)^{\ast}=B^{\ast}A^{\ast}$. –  WimC Dec 21 '12 at 19:49

1 Answer 1

up vote 1 down vote accepted

Hint for (1): $A$ has the same eigenvalues as $S A S^{-1}$ for any invertible matrix $S$. Try an $S$ that is not normal.

(2): No, $B^* = B^{-1}$ and $(AB)^* = B^* A^*$. But the statement is not true: if $A$ is hermitian and $B$ is unitary, $AB$ is not normal unless $A^2$ commutes with $B$.

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strange that is what I need to prove... –  baaa12 Dec 21 '12 at 19:51
    
Oh I forgot something I edited my question now... –  baaa12 Dec 21 '12 at 19:52
    
Are you sure there isn't some other assumption, e.g. that $A$ has eigenvalues $1$ and $-1$? –  Robert Israel Dec 21 '12 at 19:52
1  
Oh, now it's easy. In fact the product of two normal matrices that commute is normal. –  Robert Israel Dec 21 '12 at 19:53
    
Yes thank you for Question2 but still cant understand 1 –  baaa12 Dec 21 '12 at 19:58

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