10.65 Relative assassin
Discussion of relative assassins. Let R \to S be a ring map. Let N be an S-module. In this situation we can introduce the following sets of primes \mathfrak q of S:
A: with \mathfrak p = R \cap \mathfrak q we have that \mathfrak q \in \text{Ass}_ S(N \otimes _ R \kappa (\mathfrak p)),
A': with \mathfrak p = R \cap \mathfrak q we have that \mathfrak q is in the image of \text{Ass}_{S \otimes \kappa (\mathfrak p)}(N \otimes _ R \kappa (\mathfrak p)) under the canonical map \mathop{\mathrm{Spec}}(S \otimes _ R \kappa (\mathfrak p)) \to \mathop{\mathrm{Spec}}(S),
A_{fin}: with \mathfrak p = R \cap \mathfrak q we have that \mathfrak q \in \text{Ass}_ S(N/\mathfrak pN),
A'_{fin}: for some prime \mathfrak p' \subset R we have \mathfrak q \in \text{Ass}_ S(N/\mathfrak p'N),
B: for some R-module M we have \mathfrak q \in \text{Ass}_ S(N \otimes _ R M), and
B_{fin}: for some finite R-module M we have \mathfrak q \in \text{Ass}_ S(N \otimes _ R M).
Let us determine some of the relations between these sets.
Lemma 10.65.1. Let R \to S be a ring map. Let N be an S-module. Let A, A', A_{fin}, B, and B_{fin} be the subsets of \mathop{\mathrm{Spec}}(S) introduced above.
We always have A = A'.
We always have A_{fin} \subset A, B_{fin} \subset B, A_{fin} \subset A'_{fin} \subset B_{fin} and A \subset B.
If S is Noetherian, then A = A_{fin} and B = B_{fin}.
If N is flat over R, then A = A_{fin} = A'_{fin} and B = B_{fin}.
If R is Noetherian and N is flat over R, then all of the sets are equal, i.e., A = A' = A_{fin} = A'_{fin} = B = B_{fin}.
Proof.
Some of the arguments in the proof will be repeated in the proofs of later lemmas which are more precise than this one (because they deal with a given module M or a given prime \mathfrak p and not with the collection of all of them).
Proof of (1). Let \mathfrak p be a prime of R. Then we have
\text{Ass}_ S(N \otimes _ R \kappa (\mathfrak p)) = \text{Ass}_{S/\mathfrak pS}(N \otimes _ R \kappa (\mathfrak p)) = \text{Ass}_{S \otimes _ R \kappa (\mathfrak p)}(N \otimes _ R \kappa (\mathfrak p))
the first equality by Lemma 10.63.14 and the second by Lemma 10.63.16 part (1). This prove that A = A'. The inclusion A_{fin} \subset A'_{fin} is clear.
Proof of (2). Each of the inclusions is immediate from the definitions except perhaps A_{fin} \subset A which follows from Lemma 10.63.16 and the fact that we require \mathfrak p = R \cap \mathfrak q in the formulation of A_{fin}.
Proof of (3). The equality A = A_{fin} follows from Lemma 10.63.16 part (3) if S is Noetherian. Let \mathfrak q = (g_1, \ldots , g_ m) be a finitely generated prime ideal of S. Say z \in N \otimes _ R M is an element whose annihilator is \mathfrak q. We may pick a finite submodule M' \subset M such that z is the image of z' \in N \otimes _ R M'. Then \text{Ann}_ S(z') \subset \mathfrak q = \text{Ann}_ S(z). Since N \otimes _ R - commutes with colimits and since M is the directed colimit of finite R-modules we can find M' \subset M'' \subset M such that the image z'' \in N \otimes _ R M'' is annihilated by g_1, \ldots , g_ m. Hence \text{Ann}_ S(z'') = \mathfrak q. This proves that B = B_{fin} if S is Noetherian.
Proof of (4). If N is flat, then the functor N \otimes _ R - is exact. In particular, if M' \subset M, then N \otimes _ R M' \subset N \otimes _ R M. Hence if z \in N \otimes _ R M is an element whose annihilator \mathfrak q = \text{Ann}_ S(z) is a prime, then we can pick any finite R-submodule M' \subset M such that z \in N \otimes _ R M' and we see that the annihilator of z as an element of N \otimes _ R M' is equal to \mathfrak q. Hence B = B_{fin}. Let \mathfrak p' be a prime of R and let \mathfrak q be a prime of S which is an associated prime of N/\mathfrak p'N. This implies that \mathfrak p'S \subset \mathfrak q. As N is flat over R we see that N/\mathfrak p'N is flat over the integral domain R/\mathfrak p'. Hence every nonzero element of R/\mathfrak p' is a nonzerodivisor on N/\mathfrak p'. Hence none of these elements can map to an element of \mathfrak q and we conclude that \mathfrak p' = R \cap \mathfrak q. Hence A_{fin} = A'_{fin}. Finally, by Lemma 10.63.17 we see that \text{Ass}_ S(N/\mathfrak p'N) = \text{Ass}_ S(N \otimes _ R \kappa (\mathfrak p')), i.e., A'_{fin} = A.
Proof of (5). We only need to prove A'_{fin} = B_{fin} as the other equalities have been proved in (4). To see this let M be a finite R-module. By Lemma 10.62.1 there exists a filtration by R-submodules
0 = M_0 \subset M_1 \subset \ldots \subset M_ n = M
such that each quotient M_ i/M_{i-1} is isomorphic to R/\mathfrak p_ i for some prime ideal \mathfrak p_ i of R. Since N is flat we obtain a filtration by S-submodules
0 = N \otimes _ R M_0 \subset N \otimes _ R M_1 \subset \ldots \subset N \otimes _ R M_ n = N \otimes _ R M
such that each subquotient is isomorphic to N/\mathfrak p_ iN. By Lemma 10.63.3 we conclude that \text{Ass}_ S(N \otimes _ R M) \subset \bigcup \text{Ass}_ S(N/\mathfrak p_ iN). Hence we see that B_{fin} \subset A'_{fin}. Since the other inclusion is part of (2) we win.
\square
We define the relative assassin of N over S/R to be the set A = A' above. As a motivation we point out that it depends only on the fibre modules N \otimes _ R \kappa (\mathfrak p) over the fibre rings. As in the case of the assassin of a module we warn the reader that this notion makes most sense when the fibre rings S \otimes _ R \kappa (\mathfrak p) are Noetherian, for example if R \to S is of finite type.
Definition 10.65.2. Let R \to S be a ring map. Let N be an S-module. The relative assassin of N over S/R is the set
\text{Ass}_{S/R}(N) = \{ \mathfrak q \subset S \mid \mathfrak q \in \text{Ass}_ S(N \otimes _ R \kappa (\mathfrak p)) \text{ with }\mathfrak p = R \cap \mathfrak q\} .
This is the set named A in Lemma 10.65.1.
The spirit of the next few results is that they are about the relative assassin, even though this may not be apparent.
Lemma 10.65.3. Let R \to S be a ring map. Let M be an R-module, and let N be an S-module. If N is flat as R-module, then
\text{Ass}_ S(M \otimes _ R N) \supset \bigcup \nolimits _{\mathfrak p \in \text{Ass}_ R(M)} \text{Ass}_ S(N/\mathfrak pN)
and if R is Noetherian then we have equality.
Proof.
If \mathfrak p \in \text{Ass}_ R(M) then there exists an injection R/\mathfrak p \to M. As N is flat over R we obtain an injection R/\mathfrak p \otimes _ R N \to M \otimes _ R N. Since R/\mathfrak p \otimes _ R N = N/\mathfrak pN we conclude that \text{Ass}_ S(N/\mathfrak pN) \subset \text{Ass}_ S(M \otimes _ R N), see Lemma 10.63.3. Hence the right hand side is contained in the left hand side.
Write M = \bigcup M_\lambda as the union of its finitely generated R-submodules. Then also N \otimes _ R M = \bigcup N \otimes _ R M_\lambda (as N is R-flat). By definition of associated primes we see that \text{Ass}_ S(N \otimes _ R M) = \bigcup \text{Ass}_ S(N \otimes _ R M_\lambda ) and \text{Ass}_ R(M) = \bigcup \text{Ass}(M_\lambda ). Hence we may assume M is finitely generated.
Let \mathfrak q \in \text{Ass}_ S(M \otimes _ R N), and assume R is Noetherian and M is a finite R-module. To finish the proof we have to show that \mathfrak q is an element of the right hand side. First we observe that \mathfrak qS_{\mathfrak q} \in \text{Ass}_{S_{\mathfrak q}}((M \otimes _ R N)_{\mathfrak q}), see Lemma 10.63.15. Let \mathfrak p be the corresponding prime of R. Note that
(M \otimes _ R N)_{\mathfrak q} = M \otimes _ R N_{\mathfrak q} = M_{\mathfrak p} \otimes _{R_{\mathfrak p}} N_{\mathfrak q}
If \mathfrak pR_{\mathfrak p} \not\in \text{Ass}_{R_{\mathfrak p}}(M_{\mathfrak p}) then there exists an element x \in \mathfrak pR_{\mathfrak p} which is a nonzerodivisor in M_{\mathfrak p} (see Lemma 10.63.18). Since N_{\mathfrak q} is flat over R_{\mathfrak p} we see that the image of x in \mathfrak qS_{\mathfrak q} is a nonzerodivisor on (M \otimes _ R N)_{\mathfrak q}. This is a contradiction with the assumption that \mathfrak qS_{\mathfrak q} \in \text{Ass}_ S((M \otimes _ R N)_{\mathfrak q}). Hence we conclude that \mathfrak p is one of the associated primes of M.
Continuing the argument we choose a filtration
0 = M_0 \subset M_1 \subset \ldots \subset M_ n = M
such that each quotient M_ i/M_{i-1} is isomorphic to R/\mathfrak p_ i for some prime ideal \mathfrak p_ i of R, see Lemma 10.62.1. (By Lemma 10.63.4 we have \mathfrak p_ i = \mathfrak p for at least one i.) This gives a filtration
0 = M_0 \otimes _ R N \subset M_1 \otimes _ R N \subset \ldots \subset M_ n \otimes _ R N = M \otimes _ R N
with subquotients isomorphic to N/\mathfrak p_ iN. If \mathfrak p_ i \not= \mathfrak p then \mathfrak q cannot be associated to the module N/\mathfrak p_ iN by the result of the preceding paragraph (as \text{Ass}_ R(R/\mathfrak p_ i) = \{ \mathfrak p_ i\} ). Hence we conclude that \mathfrak q is associated to N/\mathfrak pN as desired.
\square
Lemma 10.65.4. Let R \to S be a ring map. Let N be an S-module. Assume N is flat as an R-module and R is a domain with fraction field K. Then
\text{Ass}_ S(N) = \text{Ass}_ S(N \otimes _ R K) = \text{Ass}_{S \otimes _ R K}(N \otimes _ R K)
via the canonical inclusion \mathop{\mathrm{Spec}}(S \otimes _ R K) \subset \mathop{\mathrm{Spec}}(S).
Proof.
Note that S \otimes _ R K = (R \setminus \{ 0\} )^{-1}S and N \otimes _ R K = (R \setminus \{ 0\} )^{-1}N. For any nonzero x \in R multiplication by x on N is injective as N is flat over R. Hence the lemma follows from Lemma 10.63.17 combined with Lemma 10.63.16 part (1).
\square
Lemma 10.65.5. Let R \to S be a ring map. Let M be an R-module, and let N be an S-module. Assume N is flat as R-module. Then
\text{Ass}_ S(M \otimes _ R N) \supset \bigcup \nolimits _{\mathfrak p \in \text{Ass}_ R(M)} \text{Ass}_{S \otimes _ R \kappa (\mathfrak p)}(N \otimes _ R \kappa (\mathfrak p))
where we use Remark 10.18.5 to think of the spectra of fibre rings as subsets of \mathop{\mathrm{Spec}}(S). If R is Noetherian then this inclusion is an equality.
Proof.
This is equivalent to Lemma 10.65.3 by Lemmas 10.63.14, 10.39.7, and 10.65.4.
\square
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