On pages 255-256 of Nocedal & Wright's Numerical Optimization (2nd edition), the authors state that the Jacobians
$J(x)$ have their singular values uniformly bounded away from zero in the region of interest; that is, there is a constantγ $\gamma> 0$ such that $\|J(x)z\| \geq \gamma\|z\|$.
Does anyone happen to know how they have gotten to the expression $\|J(x)z\| \geq \gamma\|z\|$?