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SOME BASIC RESULTS ON ACTIONS OF NON AFFINE ALGEBRAIC GROUPS

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Niveau: Supérieur, Doctorat, Bac+8
SOME BASIC RESULTS ON ACTIONS OF NON-AFFINE ALGEBRAIC GROUPS MICHEL BRION Abstract. We study actions of connected algebraic groups on normal algebraic varieties, and show how to reduce them to actions of affine subgroups. 0. Introduction Algebraic group actions have been extensively studied under the as- sumption that the acting group is affine or, equivalently, linear; see [KSS, MFK, Su]. In contrast, little seems to be known about actions of non-affine algebraic groups. In this paper, we show that these actions can be reduced to actions of affine subgroup schemes, in the setting of normal varieties. Our starting point is the following theorem of Nishi and Matsumura (see [Ma]). Let G be a connected algebraic group of automorphisms of a nonsingular algebraic variety X and denote by ?X : X ?? A(X) the Albanese morphism, that is, the universal morphism to an abelian variety (see [Se2]). Then G acts on A(X) by translations, compatibly with its action on X, and the kernel of the induced homomorphism G? A(X) is affine. Applied to the case of G acting on itself via left multiplication, this shows that the Albanese morphism ?G : G ?? A(G) is a surjective group homomorphism having an affine kernel.

  • group

  • normal gaff -equivariant

  • albanese morphism

  • any normal

  • algebraic group

  • variety

  • gaff

  • acting


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SOME BASIC RESULTS ON ACTIONS OF NON-AFFINE ALGEBRAIC GROUPS
MICHEL BRION
Abstract.We study actions of connected algebraic groups on normal algebraic varieties, and show how to reduce them to actions of ane subgroups.
0.Introduction
Algebraic group actions have been extensively studied under the as-sumption that the acting group is ane or, equivalently, linear; see [KSS, MFK, Su]. In contrast, little seems to be known about actions of non-ane algebraic groups. In this paper, we show that these actions can be reduced to actions of ane subgroup schemes, in the setting of normal varieties. Our starting point is the following theorem of Nishi and Matsumura (see [Ma]). LetGbe a connected algebraic group of automorphisms of a nonsingular algebraic varietyXand denote by
X:X →A(X) theAlbanese morphism, that is, the universal morphism to an abelian variety (see [Se2]). ThenGacts onA(X)by translations, compatibly with its action onXand the kernel of the induced homomorphism, GA(X)is ane. Applied to the case ofGacting on itself via left multiplication, this shows that the Albanese morphism
G:G →A(G) isasurjectivegrouphomomorphismhavingananekernel.Since this kernel is easily seen to be smooth and connected, this implies Chevalley’s structure theorem:any connected algebraic groupGis an extension of an abelian varietyA(G)noentcdeaenlaegbraiccayb groupGa (see [Co] for a modern proof). The Nishi–Matsumura theorem may be reformulated as follows: for any faithful action ofGon a nonsingular varietyX, the induced homo-morphismGA(X) factors through a homomorphismA(G)A(X) havinga nitekernel(see[Ma]again).Thiseasilyimpliestheexistence of aG-equivariant morphism
 :X →A, 1
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MICHEL BRION
whereAis an abelian variety which is the quotient ofA(G nya)beit subgroup scheme (see Section 3 for details). SinceA(G)=G/Ga, we haveA=G/HwhereHis a closed subgroup scheme ofGcontaining Ga withH/Ga  particular, nite; inHis ane, normalized byG, and uniquely determined byA there is a. ThenG-equivariant isomorphism X=GHY,
wheretheright-handsidedenotesthehomogeneous berbundleover G/Hassociated to the scheme-theoretic b erYof at the base point. In particular, given a faithful action of an abelian varietyAon a nonsingular varietyX, there exist a positive integernand a closed An-stable subschemeYXsuch thatX=AAnY, whereAnA denotes the kernel of the multiplication byn. For free actions (that is, abelian torsors), this result is due to Serre, see [Se1, Prop. 17]. Next, consider a faithful action ofGon a possibly singular variety X besides the Albanese morphism, we have the. Then,Albanese map
X,r:X →A(X)r, i.e., the universal rational map to an abelian variety. Moreover, the regular locusUXisG-stable, andA(U) =A(U)r=A(X)r. Thus, Gacts onA(X)rvia a homomorphismA(G)A(X)rsuch that the canonical homomorphism
hX:A(X)r →A(X) is equivariant;hXis surjective, but generally not an isomorphism, see [Se2] again. Applying the Nishi–Matsumura theorem toU, we see that the kernel of theG-action onA(X)r there exists ais ane, andG-equivariant rational map r:X →Afor some abelian varietyA as above. However,Gmay well act trivially onA(X); then there exists no morphism as above, and henceXadmits no equivariant embedding into a nonsingularG happens indeed for several classes-variety. This of examples constructed by Raynaud, see [Ra, XII 1.2, XIII 3.2] or Examples 5.2, 5.3, 5.4; in the latter example,Xis normal, andGis an abelian variety acting freely. Yet we shall show that such an equivariant embedding (in particular, such a morphism ) existslocallyfor any normalG-variety. To state our results in a precise way, we introduce some notation and conventions. We consider algebraic varieties and schemes over an algebraically closed eldk; morphisms are understood to bek-morphisms. By a variety, we mean a separated integral scheme of nite type overk a general; a point will always mean a closed point. As reference for algebraic geometry, we use the book [Ha], and [DG] for algebraic groups.