Let G be a finite group and let f be an element of order 3 in Aut(G), such that f(x)=x, implies x=e. Prove that for every prime factor p of o(G), the p-Sylow subgroup of G is normal in G. This is problem 19 in section 2.12 of Herstein's topics in algebra. I tried showing that f maps N(P) to G, but I don't really know if or how it can be done.