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A Classification of Nilpotent Orbits in Infinitesimal Symmetric Spaces

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A Classification of Nilpotent Orbits in Infinitesimal Symmetric Spaces A classification of nilpotent orbits in infinitesimal symmetric spaces Joseph A. Fox Salem State College, Mathematics Department, Salem, MA 01970 Abstract Let G be a semisimple algebraic group defined over an algebraically closed field k whose characteristic is very good for G and not equal to 2. Suppose θ is an involution on G. We also denote the induced involution on g by θ. Let K = fg 2 G : θ(g) = gg and let p be the −1-eigenspace of θ in g. The adjoint action of G on g induces an action of K on p and on the variety N (p), which consists of the nilpotent elements in p. In this paper, we give a classification of the K-orbits in N (p). To do so, we use the theory of associated cocharacters developed by Pommerening. Key words: algebraic groups, nilpotent orbits, good characteristic, involutions, associated cocharacters 1 Introduction Suppose G is a reductive linear algebraic group defined over an algebraically closed field k with g = Lie(G). The nullcone of g is the algebraic variety consisting of the nilpotent elements of g and is denoted by N (g). The adjoint action of G on g (denoted here by g · x for g 2 G and x 2 g) induces an action of G on N (g), and the G-orbits in N (g) are called nilpotent orbits. Reductive groups always have only finitely many nilpotent orbits, regardless of the value of char(k). Nilpotent orbits have many applications in representation theory and have been extensively studied (cf. [9] and [5]). Email address: [email protected] (Joseph A. Fox). Preprint submitted to Elsevier Science 28 December 2009 1.1 Background When char(k) = 0 or char(k) 0, the Jacobson{Morozov Theorem says that for each e 2 N (g), there exist a semisimple element h 2 g and a nilpotent element f 2 g such that (h; e; f) forms a basis for a copy of sl2(k) in g. Such triples are called standard triples and are crucial to the classification of the orbit set N (g)=G given by Bala and Carter in [1,2]. Given a nilpotent orbit O with e 2 O, the Jacobson{Morozov Theorem gives a standard triple (h; e; f) as noted above. From this, we can construct a parabolic subalgebra L q of g defined by q = i≥0 gi, where gi = fx 2 g :[h; x] = ixg. Bala and Carter define the notions of distinguished elements and parabolic subalgebras. These will be analogous to the featured elements and parabolic subalgebras defined in Section 3. The Bala{Carter classification shows that there is a one- to-one correspondence from the set N (g)=G to the set of G-conjugacy classes of pairs (r; q), where r is a Levi subalgebra and q is a distinguished parabolic subalgebra of r, constructed as above using a standard triple. In this paper we consider groups defined over fields of good or very good char- acteristics, defined as follows. Suppose G is quasisimple. A characteristic of 0 is considered good and very good for G. The following primes, which depend on the Cartan type of G, are also good: all primes are good if G is of Type An, all primes greater than 2 are good for Types Bn,Cn, and Dn, and all primes greater than 3 are good for the exceptional types, except for Type E8, for which primes greater than 5 are good. A good prime p is very good for G if G is not of Type An or if G is of Type An and p does not divide n + 1. For an arbitrary reductive group G, which is an almost direct product of quasisimple groups Gi and a torus T , char(k) is good (resp., very good) for G if it is good (resp., very good) for each Gi. In [15,16], Pommerening extended the Bala{Carter classification to reductive groups defined over fields of good characteristic. However, instead of using standard triples, which are not always available when char(k) is good, Pom- merening used objects called associated cocharacters, which are defined in Section 2. He was able to show that the classification of N (g)=G given by Bala and Carter remains the same when char(k) is good. Now suppose G is equipped with an involution θ, i.e., an automorphism of order 2. Let K = fg 2 G : θ(g) = gg, and for the sake of convenience let θ also denote the differential of θ, which is an involution of g. Let k be the +1-eigenspace of θ in g, and let p be the −1-eigenspace. The adjoint action of G on g induces an action of K on p. The subgroup K also acts on the nullcone N (p) of p, which is defined to be the variety N (p) = N (g) \ p. The space p is called an infinitesimal symmetric space, terminology inspired by the role that −1-eigenspaces of Cartan involutions play in the theory of Riemannian 2 symmetric spaces. Kostant and Rallis gave an extensive study of the action of K on p in [11] when G is a complex reductive group. Recently, Levy extended many of Kostant and Rallis' results to fields of good characteristic (see [12]). In particular, he shows that each irreducible component of N (p) has a dense open orbit and gives a new proof of the number of such components. He also gives results related to k[p]k, the ring of k-invariant polynomials on p. The following proposition (which is also true for a larger class of reductive groups, including G = GLn(k) when char(k) 6= 2) will be important in Section 3. Proposition 1 When G is a semisimple group defined over an algebraically closed field with a characteristic which is very good for G and not 2, there are only finitely many K-orbits in N (p). This statement follows immediately from a result of Richardson ([18, Propo- sition 7.4]) which says that there are only finitely many K-orbits in U(P ) (U(P ) being the set of unipotent elements in P ) and a result of Bardsley and Richardson ([3, Proposition 10.1]) which says U(P ) and N (p) are isomorphic as K-varieties. This isomorphism follows from Springer's result in [19] that U(G) and N (g) are isomorphic as G-varieties when char(k) is good for G. If char(k) = 0 or char(k) 0, then for any e 2 N (p) one can use the Jacobson{Morozov Theorem to obtain a standard triple (h; e; f) in g with the additional properties that h 2 k and f 2 p. A standard triple with these properties is called a normal triple. Normal triples were used by No¨elin [14] to give a classification of the orbit set N (p)=K when char(k) = 0. When char(k) = 0, normal triples play a part in the study of N (p)=K analo- gous to the part played by standard triples in the study of N (g)=G. A goal of this paper is to construct cocharacters that will replace normal triples when char(k) is very good and not 2, similar to the way in which Pommerening's associated cocharacters replaced standard triples. These cocharacters are con- structed in Section 2, and in Section 3, we will then use them to obtain a classification of N (p)=K that will hold whenever char(k) is very good and not 2. 1.2 Assumptions and Conventions Throughout, we will make the following assumptions. The group G is a semisim- ple linear algebraic group defined over an algebraically closed field k whose characteristic is very good for G and not 2. Actually, the results contained 3 herein hold even when G = GLn(k) and char(k) 6= 2. Note that in this case when char(k) divides n, G is isomorphic to a Levi subgroup of SLn+1(k), for which char(k) is very good. Any references to topological concepts refer to the Zariski topology on G and g = Lie(G). The group of units of k will be denoted k∗, and the identity component of G will be denoted by G◦. The derived subgroup of an algebraic group H will be denoted DH. There is a bijection from the set of nilpotent G-orbits in g to the set of nilpotent G=Z(G)-orbits in Lie(G=Z(G)) (see [9, Proposition 2.7a]). Since G=Z(G) is of adjoint type, we may thus assume that G also has this property when studying nilpotent orbits. x The assumptions on G imply that Lie(CG(x)) = g for all x 2 g, where CG(x) (resp., gx) denotes the centralizer of x in G (resp., g) relative to the adjoint action. Thus, the tangent space Tx(G · x) at x to the orbit G · x is equal to [g; x] (see [9, Section 2.2]). Since involutions are semisimple automorphisms when char(k) 6= 2, G contains a θ-stable Borel subgroup B which in turn contains a θ-stable torus T (see [20, Theorem 7.5]). The Lie algebra h of T is a θ-stable Cartan subalgebra of g. 2 A Replacement for Normal Triples In this section, we first state some basic facts about characters and cocharac- ters. Then we develop a special cocharacter that will serve as a replacement for normal triples when we classify N (p)=K in the next section.
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