Subrings invariant under endomorphisms
  Let S and R be the rings of regular functions on affine algebraic varieties over a field of characteristic 0, R be embedded as a subring in S, and F : S --> S be an endomorphism such that F(R) subset R. Suppose that every ideal of height 1 in R generates a proper ideal in S, and the spectrum of R has no selfintersection points. We show that if F is an automorphism so is F|_R : R --> R. When R and S have the same transcendence degree then the fact that F|_R is an automorphisms implies that F is an automorphism. 
Shulim Kaliman