Before revision:  one is on characterizations of linear systems for which the robust instability radius can be calculated exactly  and the other is on the exact instability margin for a class of nonlinear systems by properly taking the change of the equilibrium point into account .
After revision:  one is on characterizations of linear systems for which the robust instability radius can be calculated exactly and the other is a numerically tractable procedure for calculating the exact instability margin for a class of nonlinear systems by properly taking the change of the equilibrium point into account .