I am working on below statement: A is an $m \times n$ matrix, and $\lambda$ is an eigenvalue of $(A^T)A$ which eigenvector $X$ doesn't equal to zero. Show that $\lambda$ is greater than or equal to zero. I started from computing $||AX||^2$, then ended up with $\lambda\cdot (X^T)\cdot X$ is greater than or equal to zero. Then how can I know $(X^T)\cdot X$ is greater than zero?
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