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How to prove the equation below:

$(A^TA + \mu I)^{-1}A^T$ = $A^T(AA^T + \mu I)^{-1}$

no ideas at all, i major in signal processing and it needs some math techniques so i am not familiar with this part of knowledge, really need help please.

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    Have you tried expanding the equation out into two terms? For example, $(A+B)C = AC + BC$.2017-02-14
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    Needn't he use the inverse law first2017-02-14
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    At least we know that $(A^TA + \mu I)^{-1}$ is symmetric.2017-02-14
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    It is very simple! Left multiply by $(A^TA + \mu I)$, then right multiply by $(AA^T + \mu I)$, you end up with $A^T(AA^T + \mu I)=(AA^T + \mu I)A^T$ which is evidently true by development of LHS and RHS.2017-02-14

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