Lemma 39.7.1. If (G, m) is a group scheme over a field k, then the multiplication map m : G \times _ k G \to G is open.
39.7 Properties of group schemes over a field
In this section we collect some properties of group schemes over a field. In the case of group schemes which are (locally) algebraic over a field we can say a lot more, see Section 39.8.
Proof. The multiplication map is isomorphic to the projection map \text{pr}_0 : G \times _ k G \to G because the diagram
is commutative with isomorphisms as horizontal arrows. The projection is open by Morphisms, Lemma 29.23.4. \square
Lemma 39.7.2. If (G, m) is a group scheme over a field k. Let U \subset G open and T \to G a morphism of schemes. Then the image of the composition T \times _ k U \to G \times _ k G \to G is open.
Proof. For any field extension K/k the morphism G_ K \to G is open (Morphisms, Lemma 29.23.4). Every point \xi of T \times _ k U is the image of a morphism (t, u) : \mathop{\mathrm{Spec}}(K) \to T \times _ k U for some K. Then the image of T_ K \times _ K U_ K = (T \times _ k U)_ K \to G_ K contains the translate t \cdot U_ K which is open. Combining these facts we see that the image of T \times _ k U \to G contains an open neighbourhood of the image of \xi . Since \xi was arbitrary we win. \square
Lemma 39.7.3. Let G be a group scheme over a field. Then G is a separated scheme.
Proof. Say S = \mathop{\mathrm{Spec}}(k) with k a field, and let G be a group scheme over S. By Lemma 39.6.1 we have to show that e : S \to G is a closed immersion. By Morphisms, Lemma 29.20.2 the image of e : S \to G is a closed point of G. It is clear that \mathcal{O}_ G \to e_*\mathcal{O}_ S is surjective, since e_*\mathcal{O}_ S is a skyscraper sheaf supported at the neutral element of G with value k. We conclude that e is a closed immersion by Schemes, Lemma 26.24.2. \square
Lemma 39.7.4. Let G be a group scheme over a field k. Then
every local ring \mathcal{O}_{G, g} of G has a unique minimal prime ideal,
there is exactly one irreducible component Z of G passing through e, and
Z is geometrically irreducible over k.
Proof. For any point g \in G there exists a field extension K/k and a K-valued point g' \in G(K) mapping to g. If we think of g' as a K-rational point of the group scheme G_ K, then we see that \mathcal{O}_{G, g} \to \mathcal{O}_{G_ K, g'} is a faithfully flat local ring map (as G_ K \to G is flat, and a local flat ring map is faithfully flat, see Algebra, Lemma 10.39.17). The result for \mathcal{O}_{G_ K, g'} implies the result for \mathcal{O}_{G, g}, see Algebra, Lemma 10.30.5. Hence in order to prove (1) it suffices to prove it for k-rational points g of G. In this case translation by g defines an automorphism G \to G which maps e to g. Hence \mathcal{O}_{G, g} \cong \mathcal{O}_{G, e}. In this way we see that (2) implies (1), since irreducible components passing through e correspond one to one with minimal prime ideals of \mathcal{O}_{G, e}.
In order to prove (2) and (3) it suffices to prove (2) when k is algebraically closed. In this case, let Z_1, Z_2 be two irreducible components of G passing through e. Since k is algebraically closed the closed subscheme Z_1 \times _ k Z_2 \subset G \times _ k G is irreducible too, see Varieties, Lemma 33.8.4. Hence m(Z_1 \times _ k Z_2) is contained in an irreducible component of G. On the other hand it contains Z_1 and Z_2 since m|_{e \times G} = \text{id}_ G and m|_{G \times e} = \text{id}_ G. We conclude Z_1 = Z_2 as desired. \square
Remark 39.7.5. Warning: The result of Lemma 39.7.4 does not mean that every irreducible component of G/k is geometrically irreducible. For example the group scheme \mu _{3, \mathbf{Q}} = \mathop{\mathrm{Spec}}(\mathbf{Q}[x]/(x^3 - 1)) over \mathbf{Q} has two irreducible components corresponding to the factorization x^3 - 1 = (x - 1)(x^2 + x + 1). The first factor corresponds to the irreducible component passing through the identity, and the second irreducible component is not geometrically irreducible over \mathop{\mathrm{Spec}}(\mathbf{Q}).
Lemma 39.7.6. Let G be a group scheme over a perfect field k. Then the reduction G_{red} of G is a closed subgroup scheme of G.
Proof. Omitted. Hint: Use that G_{red} \times _ k G_{red} is reduced by Varieties, Lemmas 33.6.3 and 33.6.7. \square
Lemma 39.7.7. Let k be a field. Let \psi : G' \to G be a morphism of group schemes over k. If \psi (G') is open in G, then \psi (G') is closed in G.
Proof. Let U = \psi (G') \subset G. Let Z = G \setminus \psi (G') = G \setminus U with the reduced induced closed subscheme structure. By Lemma 39.7.2 the image of
is open (the first arrow is surjective). On the other hand, since \psi is a homomorphism of group schemes, the image of Z \times _ k G' \to G is contained in Z (because translation by \psi (g') preserves U for all points g' of G'; small detail omitted). Hence Z \subset G is an open subset (although not necessarily an open subscheme). Thus U = \psi (G') is closed. \square
Lemma 39.7.8. Let i : G' \to G be an immersion of group schemes over a field k. Then i is a closed immersion, i.e., i(G') is a closed subgroup scheme of G.
Proof. To show that i is a closed immersion it suffices to show that i(G') is a closed subset of G. Let k \subset k' be a perfect extension of k. If i(G'_{k'}) \subset G_{k'} is closed, then i(G') \subset G is closed by Morphisms, Lemma 29.25.12 (as G_{k'} \to G is flat, quasi-compact and surjective). Hence we may and do assume k is perfect. We will use without further mention that products of reduced schemes over k are reduced. We may replace G' and G by their reductions, see Lemma 39.7.6. Let \overline{G'} \subset G be the closure of i(G') viewed as a reduced closed subscheme. By Varieties, Lemma 33.24.1 we conclude that \overline{G'} \times _ k \overline{G'} is the closure of the image of G' \times _ k G' \to G \times _ k G. Hence
as m is continuous. It follows that \overline{G'} \subset G is a (reduced) closed subgroup scheme. By Lemma 39.7.7 we see that i(G') \subset \overline{G'} is also closed which implies that i(G') = \overline{G'} as desired. \square
Lemma 39.7.9. Let G be a group scheme over a field k. If G is irreducible, then G is quasi-compact.
Proof. Suppose that K/k is a field extension. If G_ K is quasi-compact, then G is too as G_ K \to G is surjective. By Lemma 39.7.4 we see that G_ K is irreducible. Hence it suffices to prove the lemma after replacing k by some extension. Choose K to be an algebraically closed field extension of very large cardinality. Then by Varieties, Lemma 33.14.2, we see that G_ K is a Jacobson scheme all of whose closed points have residue field equal to K. In other words we may assume G is a Jacobson scheme all of whose closed points have residue field k.
Let U \subset G be a nonempty affine open. Let g \in G(k). Then gU \cap U \not= \emptyset . Hence we see that g is in the image of the morphism
Since the image of this morphism is open (Lemma 39.7.1) we see that the image is all of G (because G is Jacobson and closed points are k-rational). Since U is affine, so is U \times _{\mathop{\mathrm{Spec}}(k)} U. Hence G is the image of a quasi-compact scheme, hence quasi-compact. \square
Lemma 39.7.10. Let G be a group scheme over a field k. If G is connected, then G is irreducible.
Proof. By Varieties, Lemma 33.7.14 we see that G is geometrically connected. If we show that G_ K is irreducible for some field extension K/k, then the lemma follows. Hence we may apply Varieties, Lemma 33.14.2 to reduce to the case where k is algebraically closed, G is a Jacobson scheme, and all the closed points are k-rational.
Let Z \subset G be the unique irreducible component of G passing through the neutral element, see Lemma 39.7.4. Endowing Z with the reduced induced closed subscheme structure, we see that Z \times _ k Z is reduced and irreducible (Varieties, Lemmas 33.6.7 and 33.8.4). We conclude that m|_{Z \times _ k Z} : Z \times _ k Z \to G factors through Z. Hence Z becomes a closed subgroup scheme of G.
To get a contradiction, assume there exists another irreducible component Z' \subset G. Then Z \cap Z' = \emptyset by Lemma 39.7.4. By Lemma 39.7.9 we see that Z is quasi-compact. Thus we may choose a quasi-compact open U \subset G with Z \subset U and U \cap Z' = \emptyset . The image W of Z \times _ k U \to G is open in G by Lemma 39.7.2. On the other hand, W is quasi-compact as the image of a quasi-compact space. We claim that W is closed.
Proof of the claim. Since W is quasi-compact, we see that points in the closure of W are specializations of points of W (Morphisms, Lemma 29.6.5). Thus we have to show that any irreducible component Z'' \subset G of G which meets W is contained in W. As G is Jacobson and closed points are rational, Z'' \cap W has a rational point g \in Z''(k) \cap W(k) and hence Z'' = Zg. But W = m(Z \times _ k W) by construction, so Z'' \cap W \not= \emptyset implies Z'' \subset W.
By the claim W \subset G is an open and closed subset of G. Now W \cap Z' = \emptyset since otherwise by the argument given in the preceding paragraph we would get Z' = Zg for some g \in W(k). Then as Z is a subgroup we could even pick g \in U(k) which would contradict Z' \cap U = \emptyset . Hence W \subset G is a proper open and closed subset which contradicts the assumption that G is connected. \square
Proposition 39.7.11. Let G be a group scheme over a field k. There exists a canonical closed subgroup scheme G^0 \subset G with the following properties
G^0 \to G is a flat closed immersion,
G^0 \subset G is the connected component of the identity,
G^0 is geometrically irreducible, and
G^0 is quasi-compact.
Proof. Let G^0 be the connected component of the identity with its canonical scheme structure (Morphisms, Definition 29.26.3). To show that G^0 is a closed subsgroup scheme we will use the criterion of Lemma 39.4.4. The morphism e : \mathop{\mathrm{Spec}}(k) \to G factors through G^0 as we chose G^0 to be the connected component of G containing e. Since i : G \to G is an automorphism fixing e, we see that i sends G^0 into itself. By Varieties, Lemma 33.7.13 the scheme G^0 is geometrically connected over k. Thus G^0 \times _ k G^0 is connected (Varieties, Lemma 33.7.4). Thus m(G^0 \times _ k G^0) \subset G^0 set theoretically. Thus m|_{G^0 \times _ k G^0} : G^0 \times _ k G^0 \to G factors through G^0 by Morphisms, Lemma 29.26.1. Hence G^0 is a closed subgroup scheme of G. By Lemma 39.7.10 we see that G^0 is irreducible. By Lemma 39.7.4 we see that G^0 is geometrically irreducible. By Lemma 39.7.9 we see that G^0 is quasi-compact. \square
Lemma 39.7.12. Let k be a field. Let T = \mathop{\mathrm{Spec}}(A) where A is a directed colimit of algebras which are finite products of copies of k. For any scheme X over k we have |T \times _ k X| = |T| \times |X| as topological spaces.
Proof. By taking an affine open covering we reduce to the case of an affine X. Say X = \mathop{\mathrm{Spec}}(B). Write A = \mathop{\mathrm{colim}}\nolimits A_ i with A_ i = \prod _{t \in T_ i} k and T_ i finite. Then T_ i = |\mathop{\mathrm{Spec}}(A_ i)| with the discrete topology and the transition morphisms A_ i \to A_{i'} are given by set maps T_{i'} \to T_ i. Thus |T| = \mathop{\mathrm{lim}}\nolimits T_ i as a topological space, see Limits, Lemma 32.4.6. Similarly we have
by the lemma above and the fact that limits commute with limits. \square
The following lemma says that in fact we can put a “algebraic profinite family of points” in an affine open. We urge the reader to read Lemma 39.8.6 first.
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
Remark 39.7.14. If G is a group scheme over a field, is there always a quasi-compact open and closed subgroup scheme? By Proposition 39.7.11 this question is only interesting if G has infinitely many connected components (geometrically).
Lemma 39.7.15. Let G be a group scheme over a field. There exists an open and closed subscheme G' \subset G which is a countable union of affines.
Proof. Let e \in U(k) be a quasi-compact open neighbourhood of the identity element. By replacing U by U \cap i(U) we may assume that U is invariant under the inverse map. As G is separated this is still a quasi-compact set. Set
where m_ n : G \times _ k \ldots \times _ k G \to G is the n-slot multiplication map (g_1, \ldots , g_ n) \mapsto m(m(\ldots (m(g_1, g_2), g_3), \ldots ), g_ n). Each of these maps are open (see Lemma 39.7.1) hence G' is an open subgroup scheme. By Lemma 39.7.7 it is also a closed subgroup scheme. \square
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