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Through experience I've seen that the following statement holds true: "$A^TA$ is always a symmetric matrix?", where $A$ is any matrix.

However can this statement be proven/falsefied?

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    $\begingroup$ Do you know what $(A\cdot B)^T$ is? $\endgroup$ – Daniel Fischer Aug 12 '13 at 13:38
  • $\begingroup$ Clearly your definition of symmetric is not (literally) to equal its own transpose. What is your definition? $\endgroup$ – wildildildlife Aug 12 '13 at 15:14
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Ideally we've already proved both $(A^T)^T=A$ and $(AB)^T=B^T A^T$. If not, prove these first. Then $(A^T A)^T=A^T (A^T)^T =A^TA$.

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We know $(AB)^T=B^TA^T$, so $(A^TA)^T=A^T(A^T)^T=A^TA$ and hence $A^TA$ is always symmetric.

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