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Fourier and Ritz describing functions are obtained for the damped, driven Duffing equation. These results are compared to the pseudo-describing function, which is defined as the ratio of the first harmonic output amplitude obtained with a real filter to the input amplitude. The results indicate that the Ritz describing function is a better approximation to the pseudo-describing function. Since a number of important nonlinear systems can be represented or approximated by the Duffing equation, it seems that the Ritz describing function has significance as another tool in nonlinear system analysis.