Hi!,

i'm having trouble to prove that $\displaystyle f(x)$ is a vector norm if and only if A is positive definite, in the property that says $\displaystyle ||x + y|| \leq ||x|| + ||y|| $.

$\displaystyle

f(x) = \frac{(x^t Ax)^\frac{1}{2}}{2}

$

Thanks in advance.