Dear All, Suppose that I regress y on x and x^2, along with other covariates. In addition, x is endogenous, and I have an IV, say, z. I just learned that it seems incorrect to use z, and its squared term z^2, to be valid IVs for x and x^2? If this correct, does it mean that I need to find an additional IV for x^2? Thanks for any comments.