Analytic formula for leading-order nonlinear coherent response in stochastic resonance.

The response of an overdamped bistable system driven by a Gaussian white noise and perturbed by a weak monochromatic signal is studied analytically. The perturbation theory is employed to calculate the nonlinear coherent response in the leading order of the amplitude of the weak signal. Simple analytic formulas for the linear and the nonlinear responses have been derived in low noise and low-frequency regime and the results based on the derived formulas are compared with the numerical results.