In the Trust Region Method we approximate a given function $f$ by a quadratic model $q_k(p)=f(x_k)+\nabla{f(x_k)}^Tp+\frac{1}{2}p^T\nabla^2{f(x_k})p$. Now, we want to minimize the function within a trusted region so we look at:
$\min q_k(p) \text{ subject to } \|p\|\leq\Delta$,
where $\Delta >0$ is the radius of the trusted region. If we have the case $\|p\|=\Delta$, what does this practically mean?
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