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## Homework Statement

Given characteristic equation for a circuit containing a diode, I must figure out how to fit a polynomial to the curve so that the van der pol equation is obtained.

**The paper I am reading is here:**

http://www.unige.ch/math/hairer60/pres/pres_rentrop.pdf

http://www.unige.ch/math/hairer60/pres/pres_rentrop.pdf

My doubts are located on page three where the author talks about fitting the polynomial of degree three. I am trying to fill in the details as it isn't quite clear what he is actually doing.

Suggestions?

Should I plug the polynomial into the characteristic equation and manipulate it? Or is there some other technique?