## 15.25 Flatness and finiteness conditions

In this section we discuss some implications of the type “flat $+$ finite type $\Rightarrow $ finite presentation”. We will revisit this result in the chapter on flatness, see More on Flatness, Section 37.1. A first result of this type was proved in Algebra, Lemma 10.107.6.

Lemma 15.25.1. Let $R$ be a ring. Let $S = R[x_1, \ldots , x_ n]$ be a polynomial ring over $R$. Let $M$ be an $S$-module. Assume

there exist finitely many primes $\mathfrak p_1, \ldots , \mathfrak p_ m$ of $R$ such that the map $R \to \prod R_{\mathfrak p_ j}$ is injective,

$M$ is a finite $S$-module,

$M$ flat over $R$, and

for every prime $\mathfrak p$ of $R$ the module $M_{\mathfrak p}$ is of finite presentation over $S_{\mathfrak p}$.

Then $M$ is of finite presentation over $S$.

**Proof.**
Choose a presentation

\[ 0 \to K \to S^{\oplus r} \to M \to 0 \]

of $M$ as an $S$-module. Let $\mathfrak q$ be a prime ideal of $S$ lying over a prime $\mathfrak p$ of $R$. By assumption there exist finitely many elements $k_1, \ldots , k_ t \in K$ such that if we set $K' = \sum Sk_ j \subset K$ then $K'_{\mathfrak p} = K_{\mathfrak p}$ and $K'_{\mathfrak p_ j} = K_{\mathfrak p_ j}$ for $j = 1, \ldots , m$. Setting $M' = S^{\oplus r}/K'$ we deduce that in particular $M'_{\mathfrak q} = M_{\mathfrak q}$. By openness of flatness, see Algebra, Theorem 10.128.4 we conclude that there exists a $g \in S$, $g \not\in \mathfrak q$ such that $M'_ g$ is flat over $R$. Thus $M'_ g \to M_ g$ is a surjective map of flat $R$-modules. Consider the commutative diagram

\[ \xymatrix{ M'_ g \ar[r] \ar[d] & M_ g \ar[d] \\ \prod (M'_ g)_{\mathfrak p_ j} \ar[r] & \prod (M_ g)_{\mathfrak p_ j} } \]

The bottom arrow is an isomorphism by choice of $k_1, \ldots , k_ t$. The left vertical arrow is an injective map as $R \to \prod R_{\mathfrak p_ j}$ is injective and $M'_ g$ is flat over $R$. Hence the top horizontal arrow is injective, hence an isomorphism. This proves that $M_ g$ is of finite presentation over $S_ g$. We conclude by applying Algebra, Lemma 10.22.2.
$\square$

Lemma 15.25.2. Let $R \to S$ be a ring homomorphism. Assume

there exist finitely many primes $\mathfrak p_1, \ldots , \mathfrak p_ m$ of $R$ such that the map $R \to \prod R_{\mathfrak p_ j}$ is injective,

$R \to S$ is of finite type,

$S$ flat over $R$, and

for every prime $\mathfrak p$ of $R$ the ring $S_{\mathfrak p}$ is of finite presentation over $R_{\mathfrak p}$.

Then $S$ is of finite presentation over $R$.

**Proof.**
By assumption $S$ is a quotient of a polynomial ring over $R$. Thus the result follows directly from Lemma 15.25.1.
$\square$

Lemma 15.25.3. Let $R$ be a ring. Let $S = R[x_1, \ldots , x_ n]$ be a graded polynomial algebra over $R$, i.e., $\deg (x_ i) > 0$ but not necessarily equal to $1$. Let $M$ be a graded $S$-module. Assume

$R$ is a local ring,

$M$ is a finite $S$-module, and

$M$ is flat over $R$.

Then $M$ is finitely presented as an $S$-module.

**Proof.**
Let $M = \bigoplus M_ d$ be the grading on $M$. Pick homogeneous generators $m_1, \ldots , m_ r \in M$ of $M$. Say $\deg (m_ i) = d_ i \in \mathbf{Z}$. This gives us a presentation

\[ 0 \to K \to \bigoplus \nolimits _{i = 1, \ldots , r} S(-d_ i) \to M \to 0 \]

which in each degree $d$ leads to the short exact sequence

\[ 0 \to K_ d \to \bigoplus \nolimits _{i = 1, \ldots , r} S_{d - d_ i} \to M_ d \to 0. \]

By assumption each $M_ d$ is a finite flat $R$-module. By Algebra, Lemma 10.77.4 this implies each $M_ d$ is a finite free $R$-module. Hence we see each $K_ d$ is a finite $R$-module. Also each $K_ d$ is flat over $R$ by Algebra, Lemma 10.38.13. Hence we conclude that each $K_ d$ is finite free by Algebra, Lemma 10.77.4 again.

Let $\mathfrak m$ be the maximal ideal of $R$. By the flatness of $M$ over $R$ the short exact sequences above remain short exact after tensoring with $\kappa = \kappa (\mathfrak m)$. As the ring $S \otimes _ R \kappa $ is Noetherian we see that there exist homogeneous elements $k_1, \ldots , k_ t \in K$ such that the images $\overline{k}_ j$ generate $K \otimes _ R \kappa $ over $S \otimes _ R \kappa $. Say $\deg (k_ j) = e_ j$. Thus for any $d$ the map

\[ \bigoplus \nolimits _{j = 1, \ldots , t} S_{d - e_ j} \longrightarrow K_ d \]

becomes surjective after tensoring with $\kappa $. By Nakayama's lemma (Algebra, Lemma 10.19.1) this implies the map is surjective over $R$. Hence $K$ is generated by $k_1, \ldots , k_ t$ over $S$ and we win.
$\square$

Lemma 15.25.4. Let $R$ be a ring. Let $S = \bigoplus _{n \geq 0} S_ n$ be a graded $R$-algebra. Let $M = \bigoplus _{d \in \mathbf{Z}} M_ d$ be a graded $S$-module. Assume $S$ is finitely generated as an $R$-algebra, assume $S_0$ is a finite $R$-algebra, and assume there exist finitely many primes $\mathfrak p_ j$, $i = 1, \ldots , m$ such that $R \to \prod R_{\mathfrak p_ j}$ is injective.

If $S$ is flat over $R$, then $S$ is a finitely presented $R$-algebra.

If $M$ is flat as an $R$-module and finite as an $S$-module, then $M$ is finitely presented as an $S$-module.

**Proof.**
As $S$ is finitely generated as an $R$-algebra, it is finitely generated as an $S_0$ algebra, say by homogeneous elements $t_1, \ldots , t_ n \in S$ of degrees $d_1, \ldots , d_ n > 0$. Set $P = R[x_1, \ldots , x_ n]$ with $\deg (x_ i) = d_ i$. The ring map $P \to S$, $x_ i \to t_ i$ is finite as $S_0$ is a finite $R$-module. To prove (1) it suffices to prove that $S$ is a finitely presented $P$-module. To prove (2) it suffices to prove that $M$ is a finitely presented $P$-module. Thus it suffices to prove that if $S = P$ is a graded polynomial ring and $M$ is a finite $S$-module flat over $R$, then $M$ is finitely presented as an $S$-module. By Lemma 15.25.3 we see $M_{\mathfrak p}$ is a finitely presented $S_{\mathfrak p}$-module for every prime $\mathfrak p$ of $R$. Thus the result follows from Lemma 15.25.1.
$\square$

The following lemma will be improved on in More on Flatness, Proposition 37.13.10.

reference
Lemma 15.25.6. Let $A$ be a valuation ring. Let $A \to B$ be a ring map of finite type. Let $M$ be a finite $B$-module.

If $B$ is flat over $A$, then $B$ is a finitely presented $A$-algebra.

If $M$ is flat as an $A$-module, then $M$ is finitely presented as a $B$-module.

**Proof.**
We are going to use that an $A$-module is flat if and only if it is torsion free, see Lemma 15.22.10. By Algebra, Lemma 10.56.10 we can find a graded $A$-algebra $S$ with $S_0 = A$ and generated by finitely many elements in degree $1$, an element $f \in S_1$ and a finite graded $S$-module $N$ such that $B \cong S_{(f)}$ and $M \cong N_{(f)}$. If $M$ is torsion free, then we can take $N$ torsion free by replacing it by $N/N_{tors}$, see Lemma 15.22.2. Similarly, if $B$ is torsion free, then we can take $S$ torsion free by replacing it by $S/S_{tors}$. Hence in case (1), we may apply Lemma 15.25.4 to see that $S$ is a finitely presented $A$-algebra, which implies that $B = S_{(f)}$ is a finitely presented $A$-algebra. To see (2) we may first replace $S$ by a graded polynomial ring, and then we may apply Lemma 15.25.3 to conclude.
$\square$

## Comments (0)