matrix F-norm inequalities on matrix sum and product

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As you know, we have the two following inequalities for sum and product (for Frobenius norm):

$\|A+B\|_F\leq\|A\|_F+\|B\|_F$

and

$\|AB\|_F\leq\|A\|_F\|B\|_F$.

The question is, under which conditions, the inequalities both turn to equality.

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Hint. The Frobenius norm of a matrix is just the $2$-norm of its vectorization.