Lemma 39.7.13. Let $k$ be an algebraically closed field. Let $G$ be a group scheme over $k$. Assume that $G$ is Jacobson and that all closed points are $k$-rational. Let $T = \mathop{\mathrm{Spec}}(A)$ where $A$ is a directed colimit of algebras which are finite products of copies of $k$. For any morphism $f : T \to G$ there exists an affine open $U \subset G$ containing $f(T)$.

**Proof.**
Let $G^0 \subset G$ be the closed subgroup scheme found in Proposition 39.7.11. The first two paragraphs serve to reduce to the case $G = G^0$.

Observe that $T$ is a directed inverse limit of finite topological spaces (Limits, Lemma 32.4.6), hence profinite as a topological space (Topology, Definition 5.22.1). Let $W \subset G$ be a quasi-compact open containing the image of $T \to G$. After replacing $W$ by the image of $G^0 \times W \to G \times G \to G$ we may assume that $W$ is invariant under the action of left translation by $G^0$, see Lemma 39.7.2. Consider the composition

The space $\pi _0(W)$ is profinite (Topology, Lemma 5.23.9 and Properties, Lemma 28.2.4). Let $F_\xi \subset T$ be the fibre of $T \to \pi _0(W)$ over $\xi \in \pi _0(W)$. Assume that for all $\xi $ we can find an affine open $U_\xi \subset W$ with $F \subset U$. Since $\psi : T \to \pi _0(W)$ is universally closed as a map of topological spaces (Topology, Lemma 5.17.7), we can find a quasi-compact open $V_\xi \subset \pi _0(W)$ such that $\psi ^{-1}(V_\xi ) \subset f^{-1}(U_\xi )$ (easy topological argument omitted). After replacing $U_\xi $ by $U_\xi \cap \pi ^{-1}(V_\xi )$, which is open and closed in $U_\xi $ hence affine, we see that $U_\xi \subset \pi ^{-1}(V_\xi )$ and $U_\xi \cap T = \psi ^{-1}(V_\xi )$. By Topology, Lemma 5.22.4 we can find a finite disjoint union decomposition $\pi _0(W) = \bigcup _{i = 1, \ldots , n} V_ i$ by quasi-compact opens such that $V_ i \subset V_{\xi _ i}$ for some $i$. Then we see that

the right hand side of which is a finite disjoint union of affines, therefore affine.

Let $Z$ be a connected component of $G$ which meets $f(T)$. Then $Z$ has a $k$-rational point $z$ (because all residue fields of the scheme $T$ are isomorphic to $k$). Hence $Z = G^0 z$. By our choice of $W$, we see that $Z \subset W$. The argument in the preceding paragraph reduces us to the problem of finding an affine open neighbourhood of $f(T) \cap Z$ in $W$. After translation by a rational point we may assume that $Z = G^0$ (details omitted). Observe that the scheme theoretic inverse image $T' = f^{-1}(G^0) \subset T$ is a closed subscheme, which has the same type. After replacing $T$ by $T'$ we may assume that $f(T) \subset G^0$. Choose an affine open neighbourhood $U \subset G$ of $e \in G$, so that in particular $U \cap G^0$ is nonempty. We will show there exists a $g \in G^0(k)$ such that $f(T) \subset g^{-1}U$. This will finish the proof as $g^{-1}U \subset W$ by the left $G^0$-invariance of $W$.

The arguments in the preceding two paragraphs allow us to pass to $G^0$ and reduce the problem to the following: Assume $G$ is irreducible and $U \subset G$ an affine open neighbourhood of $e$. Show that $f(T) \subset g^{-1}U$ for some $g \in G(k)$. Consider the morphism

which is an open immersion (because the extension of this morphism to $G \times _ k T \to G \times _ k T$ is an isomorphism). By our assumption on $T$ we see that we have $|U \times _ k T| = |U| \times |T|$ and similarly for $G \times _ k T$, see Lemma 39.7.12. Hence the image of the displayed open immersion is a finite union of boxes $\bigcup _{i = 1, \ldots , n} U_ i \times V_ i$ with $V_ i \subset T$ and $U_ i \subset G$ quasi-compact open. This means that the possible opens $Uf(t)^{-1}$, $t \in T$ are finite in number, say $Uf(t_1)^{-1}, \ldots , Uf(t_ r)^{-1}$. Since $G$ is irreducible the intersection

is nonempty and since $G$ is Jacobson with closed points $k$-rational, we can choose a $k$-valued point $g \in G(k)$ of this intersection. Then we see that $g \in Uf(t)^{-1}$ for all $t \in T$ which means that $f(t) \in g^{-1}U$ as desired. $\square$

## Post a comment

Your email address will not be published. Required fields are marked.

In your comment you can use Markdown and LaTeX style mathematics (enclose it like `$\pi$`

). A preview option is available if you wish to see how it works out (just click on the eye in the toolbar).

Unfortunately JavaScript is disabled in your browser, so the comment preview function will not work.

All contributions are licensed under the GNU Free Documentation License.

## Comments (0)

There are also: