Hi all,

In most books i've seen, it says that:

$\displaystyle

||A||_2 = \textnormal{ Root of largest eigenvalue of } A^T A

$

If $\displaystyle A $ is symmetric and positive definite, this just becomes

equal to the largest eigenvalue. However, my teacher says that this even hold for positive semidefinite. Can anybody tell me if they agree/disagree, and also why?