I have a linear transformation T. It's easy to find the linear subspaces S that are invariant under T. How do I find non-linear sets S (e.g., sets defined by degree-2 multivariate polynomials $\{ (p_1(t_1,...,t_n),p_2(t_1,...,t_n)...)|t_1,...,t_n \in F \}$ that are invariant under T?