I have a data set which I describe by this nonlinear model: $$Y_i=\theta_1 x+2\theta_3x^2+\frac{\theta_2 x^3}{3+exp(-\theta_4x +x^2)}+\theta_5+\epsilon_i$$
$$\epsilon_i \sim N(0,\sigma^2)$$ $$i=1,..,n$$
I have problems formulating a hypotheses that the model is linear and describing the testing of that hypotheses.
Any pointers, clues?