I have a system that depends on three perturbations:
$$\frac{dx}{dt}=f(x;\epsilon,\mu,\eta)$$
The $\epsilon$ perturbation is independent of the other ones, but $\mu,\eta$ have a joint interaction I want to analyze using perturbation theory. However, most of the references I have only use perturbation with one parameter.
Is there a good reference that studies the case where you have more than one parameter? My current main reference is Holmes 1995 Introduction to Perturbation Methods