10.161 Japanese rings
In this section we begin to discuss finiteness of integral closure.
Definition 10.161.1.reference Let R be a domain with field of fractions K.
We say R is N-1 if the integral closure of R in K is a finite R-module.
We say R is N-2 or Japanese if for any finite extension L/K of fields the integral closure of R in L is finite over R.
The main interest in these notions is for Noetherian rings, but here is a non-Noetherian example.
Example 10.161.2. Let k be a field. The domain R = k[x_1, x_2, x_3, \ldots ] is N-2, but not Noetherian. The reason is the following. Suppose that R \subset L and the field L is a finite extension of the fraction field of R. Then there exists an integer n such that L comes from a finite extension L_0/k(x_1, \ldots , x_ n) by adjoining the (transcendental) elements x_{n + 1}, x_{n + 2}, etc. Let S_0 be the integral closure of k[x_1, \ldots , x_ n] in L_0. By Proposition 10.162.16 below it is true that S_0 is finite over k[x_1, \ldots , x_ n]. Moreover, the integral closure of R in L is S = S_0[x_{n + 1}, x_{n + 2}, \ldots ] (use Lemma 10.37.8) and hence finite over R. The same argument works for R = \mathbf{Z}[x_1, x_2, x_3, \ldots ].
Lemma 10.161.3. Let R be a domain. If R is N-1 then so is any localization of R. Same for N-2.
Proof.
These statements hold because taking integral closure commutes with localization, see Lemma 10.36.11.
\square
Lemma 10.161.4. Let R be a domain. Let f_1, \ldots , f_ n \in R generate the unit ideal. If each domain R_{f_ i} is N-1 then so is R. Same for N-2.
Proof.
Assume R_{f_ i} is N-2 (or N-1). Let L be a finite extension of the fraction field of R (equal to the fraction field in the N-1 case). Let S be the integral closure of R in L. By Lemma 10.36.11 we see that S_{f_ i} is the integral closure of R_{f_ i} in L. Hence S_{f_ i} is finite over R_{f_ i} by assumption. Thus S is finite over R by Lemma 10.23.2.
\square
Lemma 10.161.5. Let R be a domain. Let R \subset S be a quasi-finite extension of domains (for example finite). Assume R is N-2 and Noetherian. Then S is N-2.
Proof.
Let L/K be the induced extension of fraction fields. Note that this is a finite field extension (for example by Lemma 10.122.2 (2) applied to the fibre S \otimes _ R K, and the definition of a quasi-finite ring map). Let S' be the integral closure of R in S. Then S' is contained in the integral closure of R in L which is finite over R by assumption. As R is Noetherian this implies S' is finite over R. By Lemma 10.123.14 there exist elements g_1, \ldots , g_ n \in S' such that S'_{g_ i} \cong S_{g_ i} and such that g_1, \ldots , g_ n generate the unit ideal in S. Hence it suffices to show that S' is N-2 by Lemmas 10.161.3 and 10.161.4. Thus we have reduced to the case where S is finite over R.
Assume R \subset S with hypotheses as in the lemma and moreover that S is finite over R. Let M be a finite field extension of the fraction field of S. Then M is also a finite field extension of K and we conclude that the integral closure T of R in M is finite over R. By Lemma 10.36.16 we see that T is also the integral closure of S in M and we win by Lemma 10.36.15.
\square
Lemma 10.161.6. Let R be a Noetherian domain. If R[z, z^{-1}] is N-1, then so is R.
Proof.
Let R' be the integral closure of R in its field of fractions K. Let S' be the integral closure of R[z, z^{-1}] in its field of fractions. Clearly R' \subset S'. Since K[z, z^{-1}] is a normal domain we see that S' \subset K[z, z^{-1}]. Suppose that f_1, \ldots , f_ n \in S' generate S' as R[z, z^{-1}]-module. Say f_ i = \sum a_{ij}z^ j (finite sum), with a_{ij} \in K. For any x \in R' we can write
x = \sum h_ i f_ i
with h_ i \in R[z, z^{-1}]. Thus we see that R' is contained in the finite R-submodule \sum Ra_{ij} \subset K. Since R is Noetherian we conclude that R' is a finite R-module.
\square
Lemma 10.161.7. Let R be a Noetherian domain, and let R \subset S be a finite extension of domains. If S is N-1, then so is R. If S is N-2, then so is R.
Proof.
Omitted. (Hint: Integral closures of R in extension fields are contained in integral closures of S in extension fields.)
\square
Lemma 10.161.8. Let R be a Noetherian normal domain with fraction field K. Let L/K be a finite separable field extension. Then the integral closure of R in L is finite over R.
Proof.
Consider the trace pairing (Fields, Definition 9.20.6)
L \times L \longrightarrow K, \quad (x, y) \longmapsto \langle x, y\rangle := \text{Trace}_{L/K}(xy).
Since L/K is separable this is nondegenerate (Fields, Lemma 9.20.7). Moreover, if x \in L is integral over R, then \text{Trace}_{L/K}(x) is in R. This is true because the minimal polynomial of x over K has coefficients in R (Lemma 10.38.6) and because \text{Trace}_{L/K}(x) is an integer multiple of one of these coefficients (Fields, Lemma 9.20.3). Pick x_1, \ldots , x_ n \in L which are integral over R and which form a K-basis of L. Then the integral closure S \subset L is contained in the R-module
M = \{ y \in L \mid \langle x_ i, y\rangle \in R, \ i = 1, \ldots , n\}
By linear algebra we see that M \cong R^{\oplus n} as an R-module. Hence S \subset R^{\oplus n} is a finitely generated R-module as R is Noetherian.
\square
Example 10.161.9. Lemma 10.161.8 does not work if the ring is not Noetherian. For example consider the action of G = \{ +1, -1\} on A = \mathbf{C}[x_1, x_2, x_3, \ldots ] where -1 acts by mapping x_ i to -x_ i. The invariant ring R = A^ G is the \mathbf{C}-algebra generated by all x_ ix_ j. Hence R \subset A is not finite. But R is a normal domain with fraction field K = L^ G the G-invariants in the fraction field L of A. And clearly A is the integral closure of R in L.
The following lemma can sometimes be used as a substitute for Lemma 10.161.8 in case of purely inseparable extensions.
Lemma 10.161.10. Let R be a Noetherian normal domain with fraction field K of characteristic p > 0. Let a \in K be an element such that there exists a derivation D : R \to R with D(a) \not= 0. Then the integral closure of R in L = K[x]/(x^ p - a) is finite over R.
Proof.
After replacing x by fx and a by f^ pa for some f \in R we may assume a \in R. Hence also D(a) \in R. We will show by induction on i \leq p - 1 that if
y = a_0 + a_1x + \ldots + a_ i x^ i,\quad a_ j \in K
is integral over R, then D(a)^ i a_ j \in R. Thus the integral closure is contained in the finite R-module with basis D(a)^{-p + 1}x^ j, j = 0, \ldots , p - 1. Since R is Noetherian this proves the lemma.
If i = 0, then y = a_0 is integral over R if and only if a_0 \in R and the statement is true. Suppose the statement holds for some i < p - 1 and suppose that
y = a_0 + a_1x + \ldots + a_{i + 1} x^{i + 1},\quad a_ j \in K
is integral over R. Then
y^ p = a_0^ p + a_1^ p a + \ldots + a_{i + 1}^ pa^{i + 1}
is an element of R (as it is in K and integral over R). Applying D we obtain
(a_1^ p + 2a_2^ p a + \ldots + (i + 1)a_{i + 1}^ p a^ i)D(a)
is in R. Hence it follows that
D(a)a_1 + 2D(a) a_2 x + \ldots + (i + 1)D(a) a_{i + 1} x^ i
is integral over R. By induction we find D(a)^{i + 1}a_ j \in R for j = 1, \ldots , i + 1. (Here we use that 1, \ldots , i + 1 are invertible.) Hence D(a)^{i + 1}a_0 is also in R because it is the difference of y and \sum _{j > 0} D(a)^{i + 1}a_ jx^ j which are integral over R (since x is integral over R as a \in R).
\square
Lemma 10.161.11. A Noetherian domain whose fraction field has characteristic zero is N-1 if and only if it is N-2 (i.e., Japanese).
Proof.
This is clear from Lemma 10.161.8 since every field extension in characteristic zero is separable.
\square
Lemma 10.161.12. Let R be a Noetherian domain with fraction field K of characteristic p > 0. Then R is N-2 if and only if for every finite purely inseparable extension L/K the integral closure of R in L is finite over R.
Proof.
Assume the integral closure of R in every finite purely inseparable field extension of K is finite. Let L/K be any finite extension. We have to show the integral closure of R in L is finite over R. Choose a finite normal field extension M/K containing L. As R is Noetherian it suffices to show that the integral closure of R in M is finite over R. By Fields, Lemma 9.27.3 there exists a subextension M/M_{insep}/K such that M_{insep}/K is purely inseparable, and M/M_{insep} is separable. By assumption the integral closure R' of R in M_{insep} is finite over R. By Lemma 10.161.8 the integral closure R'' of R' in M is finite over R'. Then R'' is finite over R by Lemma 10.7.3. Since R'' is also the integral closure of R in M (see Lemma 10.36.16) we win.
\square
Lemma 10.161.13. Let R be a Noetherian domain. If R is N-1 then R[x] is N-1. If R is N-2 then R[x] is N-2.
Proof.
Assume R is N-1. Let R' be the integral closure of R which is finite over R. Hence also R'[x] is finite over R[x]. The ring R'[x] is normal (see Lemma 10.37.8), hence N-1. This proves the first assertion.
For the second assertion, by Lemma 10.161.7 it suffices to show that R'[x] is N-2. In other words we may and do assume that R is a normal N-2 domain. In characteristic zero we are done by Lemma 10.161.11. In characteristic p > 0 we have to show that the integral closure of R[x] is finite in any finite purely inseparable extension of L/K(x) where K is the fraction field of R. There exists a finite purely inseparable field extension L'/K and q = p^ e such that L \subset L'(x^{1/q}); some details omitted. As R[x] is Noetherian it suffices to show that the integral closure of R[x] in L'(x^{1/q}) is finite over R[x]. And this integral closure is equal to R'[x^{1/q}] with R \subset R' \subset L' the integral closure of R in L'. Since R is N-2 we see that R' is finite over R and hence R'[x^{1/q}] is finite over R[x].
\square
Lemma 10.161.14. Let R be a Noetherian domain. If there exists an f \in R such that R_ f is normal then
U = \{ \mathfrak p \in \mathop{\mathrm{Spec}}(R) \mid R_{\mathfrak p} \text{ is normal}\}
is open in \mathop{\mathrm{Spec}}(R).
Proof.
It is clear that the standard open D(f) is contained in U. By Serre's criterion Lemma 10.157.4 we see that \mathfrak p \not\in U implies that for some \mathfrak q \subset \mathfrak p we have either
Case I: \text{depth}(R_{\mathfrak q}) < 2 and \dim (R_{\mathfrak q}) \geq 2, and
Case II: R_{\mathfrak q} is not regular and \dim (R_{\mathfrak q}) = 1.
This in particular also means that R_{\mathfrak q} is not normal, and hence f \in \mathfrak q. In case I we see that \text{depth}(R_{\mathfrak q}) = \text{depth}(R_{\mathfrak q}/fR_{\mathfrak q}) + 1. Hence such a prime \mathfrak q is the same thing as an embedded associated prime of R/fR. In case II \mathfrak q is an associated prime of R/fR of height 1. Thus there is a finite set E of such primes \mathfrak q (see Lemma 10.63.5) and
\mathop{\mathrm{Spec}}(R) \setminus U = \bigcup \nolimits _{\mathfrak q \in E} V(\mathfrak q)
as desired.
\square
Lemma 10.161.15. Let R be a Noetherian domain. Then R is N-1 if and only if the following two conditions hold
there exists a nonzero f \in R such that R_ f is normal, and
for every maximal ideal \mathfrak m \subset R the local ring R_{\mathfrak m} is N-1.
Proof.
First assume R is N-1. Let R' be the integral closure of R in its field of fractions K. By assumption we can find x_1, \ldots , x_ n in R' which generate R' as an R-module. Since R' \subset K we can find f_ i \in R nonzero such that f_ i x_ i \in R. Then R_ f \cong R'_ f where f = f_1 \ldots f_ n. Hence R_ f is normal and we have (1). Part (2) follows from Lemma 10.161.3.
Assume (1) and (2). Let K be the fraction field of R. Suppose that R \subset R' \subset K is a finite extension of R contained in K. Note that R_ f = R'_ f since R_ f is already normal. Hence by Lemma 10.161.14 the set of primes \mathfrak p' \in \mathop{\mathrm{Spec}}(R') with R'_{\mathfrak p'} non-normal is closed in \mathop{\mathrm{Spec}}(R'). Since \mathop{\mathrm{Spec}}(R') \to \mathop{\mathrm{Spec}}(R) is closed the image of this set is closed in \mathop{\mathrm{Spec}}(R). For such a ring R' denote Z_{R'} \subset \mathop{\mathrm{Spec}}(R) this image.
Pick a maximal ideal \mathfrak m \subset R. Let R_{\mathfrak m} \subset R_{\mathfrak m}' be the integral closure of the local ring in K. By assumption this is a finite ring extension. By Lemma 10.36.11 we can find finitely many elements x_1, \ldots , x_ n \in K integral over R such that R_{\mathfrak m}' is generated by x_1, \ldots , x_ n over R_{\mathfrak m}. Let R' = R[x_1, \ldots , x_ n] \subset K. With this choice it is clear that \mathfrak m \not\in Z_{R'}.
As \mathop{\mathrm{Spec}}(R) is quasi-compact, the above shows that we can find a finite collection R \subset R'_ i \subset K such that \bigcap Z_{R'_ i} = \emptyset . Let R' be the subring of K generated by all of these. It is finite over R. Also Z_{R'} = \emptyset . Namely, every prime \mathfrak p' lies over a prime \mathfrak p'_ i such that (R'_ i)_{\mathfrak p'_ i} is normal. This implies that R'_{\mathfrak p'} = (R'_ i)_{\mathfrak p'_ i} is normal too. Hence R' is normal, in other words R' is the integral closure of R in K.
\square
Lemma 10.161.16 (Tate).reference Let R be a ring. Let x \in R. Assume
R is a normal Noetherian domain,
R/xR is a domain and N-2,
R \cong \mathop{\mathrm{lim}}\nolimits _ n R/x^ nR is complete with respect to x.
Then R is N-2.
Proof.
We may assume x \not= 0 since otherwise the lemma is trivial. Let K be the fraction field of R. If the characteristic of K is zero the lemma follows from (1), see Lemma 10.161.11. Hence we may assume that the characteristic of K is p > 0, and we may apply Lemma 10.161.12. Thus given L/K a finite purely inseparable field extension we have to show that the integral closure S of R in L is finite over R.
Let q be a power of p such that L^ q \subset K. By enlarging L if necessary we may assume there exists an element y \in L such that y^ q = x. Since R \to S induces a homeomorphism of spectra (see Lemma 10.46.7) there is a unique prime ideal \mathfrak q \subset S lying over the prime ideal \mathfrak p = xR. It is clear that
\mathfrak q = \{ f \in S \mid f^ q \in \mathfrak p\} = yS
since y^ q = x. Observe that R_{\mathfrak p} is a discrete valuation ring by Lemma 10.119.7. Then S_{\mathfrak q} is Noetherian by Krull-Akizuki (Lemma 10.119.12). Whereupon we conclude S_{\mathfrak q} is a discrete valuation ring by Lemma 10.119.7 once again. By Lemma 10.119.10 we see that \kappa (\mathfrak q)/\kappa (\mathfrak p) is a finite field extension. Hence the integral closure S' \subset \kappa (\mathfrak q) of R/xR is finite over R/xR by assumption (2). Since S/yS \subset S' this implies that S/yS is finite over R. Note that S/y^ nS has a finite filtration whose subquotients are the modules y^ iS/y^{i + 1}S \cong S/yS. Hence we see that each S/y^ nS is finite over R. In particular S/xS is finite over R. Also, it is clear that \bigcap x^ nS = (0) since an element in the intersection has qth power contained in \bigcap x^ nR = (0) (Lemma 10.51.4). Thus we may apply Lemma 10.96.12 to conclude that S is finite over R, and we win.
\square
Lemma 10.161.17. Let R be a ring. If R is Noetherian, a domain, and N-2, then so is R[[x]].
Proof.
Observe that R[[x]] is Noetherian by Lemma 10.31.2. Let R' \supset R be the integral closure of R in its fraction field. Because R is N-2 this is finite over R. Hence R'[[x]] is finite over R[[x]]. By Lemma 10.37.9 we see that R'[[x]] is a normal domain. Apply Lemma 10.161.16 to the element x \in R'[[x]] to see that R'[[x]] is N-2. Then Lemma 10.161.7 shows that R[[x]] is N-2.
\square
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