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Been a while since my first degree, and I can't seem to solve this kiddy-level question. Please indulge me:

A an orthogonal matrix, D a diagonal matrix.

is it true that $A^TDA = D$ ?

(where $^T$ is the transpose operator) Thanks.

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Nope. Have you tried any examples? – Qiaochu Yuan Nov 15 '11 at 20:41
I did now... thanks! – Bare Foot Nov 15 '11 at 20:46
up vote 1 down vote accepted

Not true, a simple example: $A=\left(\begin{matrix} 0& 1\\ 1&0 \end{matrix}\right)$, $D=\left(\begin{matrix} 1& 0\\ 0&2 \end{matrix}\right)$.

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