In this paper, we are concerned with the analysis of two-transistor circuits. Applying technique for the numerical verification, we prove rigorously the existence of five solutions in a two-transistor circuit. The system of equations for a transistor circuit is obtained as nonlinear equations, therefore Krawczyk's method is applied for proving the existence of a solution.
"Numerical Existence Proof of Five Solutions for Certain Two-Transistor Circuit Equations." Japan J. Indust. Appl. Math. 26 (2-3) 327 - 336, October 2009.