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 on the exact instability margin for nominal systems parametrized by a perturbation parameter .