Let $A$ be a $n\times m$ matrix, where $n \neq m$, without any zero or equal rows.
What conditions on $A$ would guarantee that
$A^T A$ is invertable?
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2$A$ having full rank is necessary. I assume also sufficient. – Aug 01 '16 at 12:51
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1http://math.stackexchange.com/questions/349738/prove-rank-ata-rank-a-for-any-a-m-times-n – Augustin Aug 01 '16 at 13:04