I've recently learned about using weighted residuals with the Galerkin method to numerically approximate even-order differential equations (for linear elements, I'm still a beginner). It seems for odd-order differential equations you cannot use integration by parts to reduce the order of the third-order differential operator.
I've read that the Petrov-Galerkin method can approximate these solutions, but I cannot find cogent material on it or examples of it in action. Can someone please explain?