Consider a nonlinear operator $\cal H$ which maps a function to a function (e.g., a map from a starting wave function $f(x,y,z)$ to a later wave function according to some nonlinear PDE) and an $\epsilon$perturbation on the input function. All realworld examples that i can think of can then be linearized (to some nonzero linear operator $\cal A$ with some order $m$) for small $\epsilon$: $$ \cal H[f+\epsilon g]\cal H[f]=\epsilon^m \cal A[g] \qquad +O(\epsilon^{m+1}) $$ Are there any realworld counterexamples which stay nonlinear in this limit? I know I haven't defined "realworld"; I'd ideally like a nonlinear PDE, but would settle for anything that comes up in some application (e.g., integral equations, or systemcontrol vectors).
