Implements digital filter based on Duffing

As we know Duffing equation works only in analog domain. But in practice it's completely usefulness, we need digital filter which works like Duffing. I try to do it with Volterra series.

Duffing - incoming phase problem

As we all know, Duffing equation go to grate scale period when incoming signal fit to some criterion (sinusoidal and amplitude of resulting signal must be greater than some constant - one can give constant from Lyapunov exponent method). In theory all seems great...