I am looking at a proof for why $A^TA$ is positive semi-definite when $A$ is $n\times n$ and it has this line. $$ v^TAA^Tv = A^Tv \cdot A^Tv ≥ 0. $$ I understand what $v^TAA^Tv$ means and the purpose of proving that it's nonnegative, etc... My problem is that I am a linear algebra novice and do not necessarily understand how the first part $v^TAA^Tv$ is equivalent to $A^Tv \cdot A^Tv$. I know that $a^Tb = a \cdot b$, but something else is going on, no? Appreciate any help!
help me understand a line in an “$A^TA$ is positive, semi-definite” proof
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linear-algebra
proof-writing