Let (R,\m) and (S,\n) be commutative Noetherian local rings, and let
Ο:RβS be a flat local homomorphism such that \m S = \n and the
induced map on residue fields R/\m \to S/\n is an isomorphism. Given a
finitely generated R-module M, we show that M has an S-module structure
compatible with the given R-module structure if and only if \Ext^i_R(S,M)=0
for each iβ₯1.
We say that an S-module N is {\it extended} if there is a finitely
generated R-module M such that Nβ SβRβM. Given a short exact
sequence 0βN1ββNβN2ββ0 of finitely generated S-modules, with
two of the three modules N1β,N,N2β extended, we obtain conditions forcing the
third module to be extended. We show that every finitely generated module over
the Henselization of R is a direct summand of an extended module, but that
the analogous result fails for the \m-adic completion.Comment: 16 pages, AMS-TeX; final version to appear in Michigan Math. J.;
corrected proof of Main Theorem and made minor editorial changes; v3 has
dedication to Mel Hochste