The Stacks project

Proof. We already know that $X \to \mathop{\mathrm{Spec}}(\mathbf{Z})$ is separated and locally of finite presentation (Lemma 108.5.2). We also know that $X \to \mathop{\mathrm{Spec}}(\mathbf{Z})$ satisfies the existence part of the valuative criterion, see Lemma 108.5.3. By the valuative criterion for properness, it suffices to prove our Quot space is quasi-compact, see Morphisms of Spaces, Lemma 67.44.1. Thus it suffices to find a quasi-compact scheme $T$ and a surjective morphism $T \to X$. Let $m$ be the integer found in Varieties, Lemma 33.35.18. Let

We will write $\mathbf{P}^ n$ for $\mathbf{P}^ n_\mathbf {Z} = \text{Proj}(\mathbf{Z}[T_0, \ldots , T_ n])$ and unadorned products will mean products over $\mathop{\mathrm{Spec}}(\mathbf{Z})$. The idea of the proof is to construct a “universal” map

over an affine scheme $T$ and show that every point of $X$ corresponds to a cokernel of this in some point of $T$.

Definition of $T$ and $\Psi $. We take $T = \mathop{\mathrm{Spec}}(A)$ where

where $i \in \{ 1, \ldots , r\} $, $j \in \{ 1, \ldots , N\} $ and $E = (e_0, \ldots , e_ n)$ runs through the multi-indices of total degree $|E| = \sum _{k = 0, \ldots n} e_ k = m$. Then we define $\Psi $ to be the map whose $(i, j)$ matrix entry is the map

where the sum is over $E$ as above (but $i$ and $j$ are fixed of course).

Consider the quotient $\mathcal{Q} = \mathop{\mathrm{Coker}}(\Psi )$ on $T \times \mathbf{P}^ n$. By More on Morphisms, Lemma 37.54.1 there exists a $t \geq 0$ and closed subschemes

such that the pullback $\mathcal{Q}_ p$ of $\mathcal{Q}$ to $(T_ p \setminus T_{p + 1}) \times \mathbf{P}^ n$ is flat over $T_ p \setminus T_{p + 1}$. Observe that we have an exact sequence

by pulling back the exact sequence defining $\mathcal{Q} = \mathop{\mathrm{Coker}}(\Psi )$. Therefore we obtain a morphism

Since the left hand side is a Noetherian scheme and the inclusion on the right hand side is open, it suffices to show that any point of $X$ is in the image of this morphism.

Let $k$ be a field and let $x \in X(k)$. Then $x$ corresponds to a surjection $\mathcal{O}_{\mathbf{P}^ n_ k}^{\oplus r} \to \mathcal{F}$ of coherent $\mathcal{O}_{\mathbf{P}^ n_ k}$-modules such that the Hilbert polynomial of $\mathcal{F}$ is $P$. Consider the short exact sequence

By Varieties, Lemma 33.35.18 and our choice of $m$ we see that $\mathcal{K}$ is $m$-regular. By Varieties, Lemma 33.35.12 we see that $\mathcal{K}(m)$ is globally generated. By Varieties, Lemma 33.35.10 and the definition of $m$-regularity we see that $H^ i(\mathbf{P}^ n_ k, \mathcal{K}(m)) = 0$ for $i > 0$. Hence we see that

by our choice of $N$. This gives a surjection

Twisting back down and using the short exact sequence above we see that $\mathcal{F}$ is the cokernel of a map

There is a unique ring map $\tau : A \to k$ such that the base change of $\Psi $ by the corresponding morphism $t = \mathop{\mathrm{Spec}}(\tau ) : \mathop{\mathrm{Spec}}(k) \to T$ is $\Psi _ x$. This is true because the entries of the $N \times r$ matrix defining $\Psi _ x$ are homogeneous polynomials $\sum \lambda _{i, j, E} T_0^{e_0} \ldots T_ n^{e_ n}$ of degree $m$ in $T_0, \ldots , T_ n$ with coefficients $\lambda _{i, j, E} \in k$ and we can set $\tau (a_{i, j, E}) = \lambda _{i, j, E}$. Then $t \in T_ p \setminus T_{p + 1}$ for some $p$ and the image of $t$ under the morphism above is $x$ as desired. $\square$