(Sorry for the mistakes first thread using hand held device) Hello, I was working on Harold T. Davis Introduction to Nonlinear Differential and Integral Equations I saw this following equations 1-So equation 4 came as a result of chain rule applies on equation. 3 ? 2- how did equation 5 come about? And specially the "2" in second term. 3-What kind of calculus I should study, so I know this sort of stuff and I can save some time. Thank you.