Conditional gain is necessary and sufficient for the robust stabilization of nonlinear systems. - Anna L. Chen; Gui-Qiang G. Chen; Randy A. Freeman (CDC, 2004)

Stability of Nonlinear Feedback Systems: A New Small-Gain Theorem. - Anna L. Chen; Gui-Qiang G. Chen; Randy A. Freeman (SIAM J. Control. Optim., 2007)