Bourbaki

Proposition 2

Tag 04G6, permanent. Cite it as [Bourbaki, Tag 04G6].

Commutative Algebra, chapter III, § 3. m-adic topologies on Noetherian rings

Let AA be a commutative Noetherian ring, mm an ideal of AA and E,FE, F two finitely generated AA-modules. If (Fn)(F_n) is an mm-good filtration on FF, the submodules HomA(E,Fn)\mathrm{Hom}_A(E, F_n) form an mm-good filtration on the AA-module HomA(E,F)\mathrm{Hom}_A(E, F). As mkFn⊂Fn+km^k F_n \subset F_{n+k} for n∈Z,k≥0n \in \mathbf{Z}, k \geq 0, it is also true that

mkHomA(E,Fn)⊂HomA(E,Fn+k); m^k \mathrm{Hom}_A(E, F_n) \subset \mathrm{Hom}_A(E, F_{n+k});

the family (HomA(E,Fn))n∈Z(\mathrm{Hom}_A(E, F_n))_{n \in \mathbf{Z}} is then a filtration on HomA(E,F)\mathrm{Hom}_A(E, F) compatible with its module structure over the ring AA filtered by the mm-adic filtration. Since EE is finitely generated, there exists an integer r>0r > 0 and a surjective AA-homomorphism u:Ar→Eu : A^r \to E which defines an injective AA-homomorphism

v=Hom(u,1F):HomA(E,F)→HomA(Ar,F); v = \mathrm{Hom}(u, 1_F) : \mathrm{Hom}_A(E, F) \to \mathrm{Hom}_A(A^r, F);

clearly vv is compatible with the filtrations (HomA(E,Fn))(\mathrm{Hom}_A(E, F_n)) and (HomA(Ar,Fn))(\mathrm{Hom}_A(A^r, F_n)). As HomA(E,F)\mathrm{Hom}_A(E, F) and HomA(Ar,F)\mathrm{Hom}_A(A^r, F) are finitely generated (Lemma 2), it is sufficient by virtue of Proposition 1 to show that the filtration (HomA(Ar,F))(\mathrm{Hom}_A(A^r, F)) is m-good; but this is immediate by virtue of the existence of the canonical isomorphism HomA(Ar,Fr)→Fnr\mathrm{Hom}_A(A^r, F_r) \to F_n^r and the fact that the relation mFn=Fn+mF_n = F_n +, implies m(Fnr)=(mFn)r=Fn+1rm(F_n^r) = (mF_n)^r = F_{n+1}^r (Algebra, Chapter II, § 3, no. 7, Remark).

Bourbaki puts the proof directly after the statement, so what is above is the statement and its proof together.

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Cites

Transcribed from the printed volume. Not translated.