On the boundedness of solutions of nonlinear integral equations

Sufficient conditions are presented for the boundedness of the solutions of a vector nonlinear Volterra integral equation of the second kind that frequently arises in the study of automatic control systems containing an arbitrary finite number of time-varying nonlinear elements. Similar conditions are given for the boundedness of the solutions of the discrete analog of the integral equation. A direct application of the results yields a Nyquist-like frequency-domain condition for the “bounded-input implies bounded-output stability” of a large class of feedback systems containing a single time-varying nonlinear element.