A new family of irreducible subgroups of the orthogonal algebraic groups
Abstract
Let and let be a simply connected, simple algebraic group of type over an algebraically closed field Also let be the subgroup of type of embedded in the usual way. In this paper, we correct an error in a proof of a theorem of Seitz (Mem. Amer. Math. Soc. 67 (1987), no. 365), resulting in the discovery of a new family of triples where denotes a finite-dimensional, irreducible, rational on which -module, acts irreducibly. We go on to investigate the impact of the existence of the new examples on the classification of the maximal closed connected subgroups of the classical algebraic groups.
1. Introduction
Let be an algebraically closed field of characteristic In the 1950s, Dynkin determined the maximal closed connected subgroups of the simple classical type linear algebraic groups defined over assuming , (see Reference 11Reference 12); in 1987, Seitz Reference 23 established an analogous classification in the case where The main step in both of these classifications is the determination of all triples . where is a simple linear algebraic group defined over , is a proper closed connected subgroup of and , is a non-trivial irreducible, finite-dimensional ( if -restricted ) on which -module acts irreducibly. The determination of these so-called “irreducible triples” is covered in the work of Dynkin Reference 11Reference 12 (in case Seitz ),Reference 23 (in case and is a classical group), and Testerman Reference 26 (in case and is of exceptional type). The existence of an irreducible triple of the form as above, arising from a rational representation indicates that , is not maximal in the smallest classical group containing both and while the large majority of tensor-indecomposable irreducible representations of a simple algebraic group give rise to maximal subgroups of the smallest classical group containing the image. ,
Recently, the second author’s PhD student Nathan Scheinmann discovered an irreducible triple which does not appear in Reference 23, Theorem 1, Table 1. Namely, take to be of characteristic 3, and embedded in the usual way in as the stabilizer of a non-singular 1-space on the 8-dimensional natural module for Consider the irreducible . with highest weight -module (here , is a set of fundamental weights for , , is the highest weight of the natural and we label Dynkin diagrams as in -module,Reference 3). The restriction of the highest weight to a maximal torus of shows the existence of a factor of highest weight -composition ( , a set of fundamental weights for , and consulting )Reference 19, one sees that these modules are both of dimension and hence , acts irreducibly on .
The absence of this example from Reference 23, Table 1 is the result of an error in the proof of Reference 23, 8.7. Here, we correct the error in this proof and, in so doing, establish the existence of a whole new family of modules for the group on which acts irreducibly. For a fixed and a fixed there are finitely many modules , but for each , there exist infinitely many primes for which there is a new example. The precise description of the family is given in Theorem 1.2 below. In addition to this infinite family, our investigations revealed one further example of an irreducibly acting subgroup which does not appear in Reference 23, Table 1, namely the group defined over a field of characteristic , acting on , the irreducible module with highest weight , has a subgroup , contained in a maximal rank subgroup of type , both acting irreducibly on , The triple . appears in Reference 23, Table 1, as well as the triple However, the triple . is omitted.
The goal of this paper is two-fold: first we concentrate on the embedding and determine all irreducible representations of -restricted whose restriction to is irreducible, thereby correcting Reference 23, 8.7; see Theorem 1.2 below. The second goal of the paper is to show that the existence of the new examples has no further influence on the main results of Reference 23 and Reference 26. Indeed, the proofs of the main theorems in these two articles depend on an inductive hypothesis, concerning the list of examples for smaller rank groups. The new family of examples for the pair as well as the one “new” example for the pair , alters the inductive hypothesis and therefore requires one to take these new examples into consideration when working through all other possible embeddings. This is precisely what has been carried out in the proofs of Proposition 1.5, Proposition 1.7, and Theorem 1.8.
Statement of results
Let be a simply connected, simple algebraic group of type over Also let be the subgroup of type embedded in in the usual way, as the stabilizer of a non-singular subspace of the natural module for -dimensional Fix a maximal torus of and a Borel subgroup containing Denote by . the corresponding set of fundamental weights for ordered as in Reference 3, where the natural -dimensional has highest weight -module Let be a graph automorphism of stabilizing and with , the group of , points. Our first main result is the following; the proof is given in Section -fixed3.
The set of weights which is listed in Reference 23, Table 1 for the pair is
So we see that the new examples are a generalization of those found by Seitz, where one congruence condition is replaced by a set of congruence conditions. (Note that there are new examples only if It is perhaps informative to point out precisely what error occurs in the proof of .)Reference 23, 8.7, where the embedding is considered. In the proof, Seitz defines a certain vector in the irreducible of highest weight -module and shows that this vector is annihilated by all simple root vectors in the Lie algebra of which then implies that , does not act irreducibly. However if satisfies the congruence conditions, the vector is in fact the zero vector in and so does not give rise to a second composition factor as claimed.
The omission of the triple from Reference 23, Table 1 is of a different nature, and occurs in the proof of Reference 23, 15.13. In the first part of the proof, there is a reduction to the case where acts irreducibly on the natural with highest weight -module and the precise embedding we have here is for , (following Seitz’s notation, is the third fundamental dominant weight), so or Then Seitz argues that . for some Since he is assuming that . is not the natural he invokes the inductive hypothesis and deduces that -module, Now Seitz concludes that . is a spin module for Evidently, he considers only the embedding of . in omitting to include the case of , in .
Returning to the family of examples described in Theorem 1.2, it is natural to ask how one might discover the given set of congruence conditions, and here we must give credit to the work of Ford in Reference 13, where he studied irreducible triples of the form , a simple classical type algebraic group over , a disconnected closed subgroup of with simple, and an irreducible on which -module acts irreducibly. He discovered a family of irreducible triples for the embedding where the highest weight of the irreducible , satisfies similar congruence conditions. His methods were later applied by Cavallin in -moduleReference 9 when studying irreducible having precisely two -modules factors. -composition
The second goal of this article is to show that the existence of the new examples for the pair described by Theorem ,1.2, and the further omitted example has no further influence on the main theorems in Reference 23Reference 26. To explain the issue which must be addressed and our approach to the problem, we must describe to some extent the strategy of the proof of Reference 23, Theorem 1. First note that the assumption that acts irreducibly on some implies that -module is semisimple. One of the main techniques used to determine the triples as above involves arguing inductively, working with a suitable embedding of parabolic subgroups, where Indeed, .Reference 23, 2.1 implies that if acts irreducibly on then the derived subgroup acts irreducibly on the commutator quotient an irreducible , Moreover, the highest weight of -module. as a is the restriction of the highest weight of -module to an appropriate maximal torus of (This is a variation of a result of .Reference 24.) Thus, Seitz and Testerman proceed by induction on the rank of Seitz treats the case ; of type by ad hoc methods, exploiting the fact that all weights of an irreducible are of multiplicity one. Now Theorem -module1.2 above introduces a new family of examples of irreducible triples. As a consequence, one needs to reinvestigate all embeddings where the pair , may arise when considering the projection of a Levi factor , of into a simple component of a Levi factor of under the additional hypothesis that , acts irreducibly on a whose highest weight has restriction to the -module of -component among the new examples described by Theorem 1.2. This is precisely what we consider in Proposition 1.5 below. A similar analysis must be carried out for the one new example as a potential embedding of Levi factors. This easier case is covered by Proposition 1.7. In order to state the results, we introduce the following terminology.
Note that the above congruence conditions are precisely those satisfied by the highest weights in Theorem 1.2 but not appearing in Reference 23, Table 1. (See the remark following the statement of Theorem 1.2.)
For the proofs of Proposition 1.5, Proposition 1.7, and Theorem 1.8, we require the following inductive hypotheses.
The classical case
Let be of classical type. The next two results ensure that, under the assumption of Hypothesis 1.4 for all embeddings with the only new examples of irreducible triples are those described in Remark 1.1.
The exceptional case
We now turn to the consideration of the case where is a simply connected, simple algebraic group of exceptional type over and is a proper closed, connected subgroup of acting irreducibly on some irreducible -restricted As usual, -module. is then semisimple, and once again, we must consider the possibility of a parabolic embedding with Levi factor of of type Levi factor , of having a simple factor of type with the action on the commutator quotient arising from a weight which satisfies the congruence conditions. (Note that the pair , will never occur as an embedding of Levi factors when is exceptional.) In particular, is of type for or .
Note that the existence of the examples arising in Theorem 1.8 had already been established by Testerman Reference 26, Main Theorem. The proof of the “only if” direction requires us to treat, eventually ruling out, several new potential configurations that arise from Theorem 1.2 in the inductive process, as explained in Section 5.
About the proofs
We conclude this section with a brief discussion of the methods and further remarks on our inductive assumption (Hypothesis 1.4). In order to prove Theorem 1.2, we first show that it is enough to work with the Lie algebras of and Indeed, as is the irreducible -restricted, -module is generated by a maximal vector for as a module for the universal enveloping algebra of Therefore in order to show that is irreducible, it suffices to show that where , is the universal enveloping algebra of We rely on the fact that any irreducible module for . is self-dual as a (see -module3.1 below), and apply the techniques developed by Ford in Reference 13, further investigated by Cavallin in Reference 9, to establish this generation result.
For the proof of Proposition 1.5, we carry out an analysis used by Seitz in Reference 23, Section 8, but applied specifically to the group He first shows that a proper closed connected subgroup . acts irreducibly on a non-trivial irreducible only if either -module acts irreducibly and tensor indecomposably on the natural module for or the triple , is known. This part of our proof is not at all original, but we include it for completeness. At this point, however, our proof proceeds along different lines; we compare the commutator series for two different parabolic embeddings and obtain conditions on the highest weight which are compatible with the given congruence conditions only if the pair is which is handled by Theorem ,1.2. The proof of Proposition 1.7 is much simpler given that we are dealing with a fixed-rank embedding.
For the proof of Theorem 1.8, we proceed differently than in Reference 26; we use the classification of the maximal closed positive-dimensional subgroups of the exceptional type algebraic groups, given in Reference 18, which was not available when Reference 26 was written. Hence, we first consider the case where is maximal, find only the two examples of the theorem and conclude using the main result of Reference 26 for the group .
In addition to Hypothesis 1.4, we rely upon two further results in Reference 23, namely Reference 23, Theorem 4.1 and Reference 23, 6.1. The first result classifies the irreducible triples when the second covers the case where , The proofs of these results are completely independent of the results in .Reference 23, Section 8. Finally, we will use the results of Reference 23, Section 2 concerning parabolic embeddings and commutator series in irreducible modules for semisimple groups.
2. Preliminaries
In this section, we introduce the notation that shall be used in the remainder of the paper, and recall some basic properties of rational modules for simple linear algebraic groups. We rely on the standard reference Reference 16 for a treatment of this general theory.
2.1. Notation
Let be an algebraically closed field of characteristic and let be a simply connected, simple linear algebraic group over (All algebraic groups considered here will be linear algebraic groups, even if we omit to say so explicitly.) Also fix a Borel subgroup of where is a maximal torus of and denotes the unipotent radical of Let and let be the corresponding base of the root system of where and denote the sets of positive and negative roots, respectively. Throughout we use the ordering of simple roots as in Reference 3. Let be the Weyl group of and for , denote by the corresponding reflection. In addition, let
denote the character group of and write for a fixed inner product on the space -invariant Also let be the fundamental dominant weights for corresponding to our choice of base that is, for where
for , Set . and call a character a dominant (or simply dominant weight, if the choice of torus is clear in the context). Finally, we say that -weight is under (and we write if ) for some We also write to indicate that and
2.2. Rational modules
In this section, we recall some elementary facts on weights and multiplicities, as well as basic properties of Weyl and irreducible modules for Let be a finite-dimensional, rational Then -module.
where, for A weight is called a weight of if in which case is said to be its corresponding weight space. Also, we denote by the multiplicity of in and let denote the set of weights of and write for the set of dominant weights of It is well known that each weight of is to a unique dominant weight in -conjugate Also, if then for every and all weights in a , have the same multiplicity. -orbit
A non-zero vector is called a maximal vector of weight for the pair if and Now for a dominant weight, we write for the Weyl module having highest weight and denote by the unique irreducible quotient of In other words,
where is the unique maximal submodule of called the radical of We write for and for Also, we denote by the induced having highest weight -module Recall that has a unique simple submodule, isomorphic to and that
where denotes the longest element in For we write to denote the number of times the irreducible -module appears as a composition factor of We also use the notation
to denote the subgroup of -root corresponding to the root (that is, is a morphism of algebraic groups inducing an isomorphism onto such that , for and Finally, we fix a Chevalley basis ). for the Lie algebra of compatible with our choice of where are root vectors for and for The proof of the following result can be deduced from applying the Poincaré-Birkhoff-Witt Theorem Reference 4 to Reference 10, A. 6.4.
We conclude this section by illustrating how Lemma 2.1 can provide information on weight multiplicities in certain irreducible in the case where -modules is of type over Consider the dominant -weights and Writing an application of Lemma 2.1 then shows that is spanned by
where is a maximal vector in for (We used the fact that for together with the commutator formula.) Finally, we set
3. Proof of Theorem 1.2
Let be an algebraically closed field having characteristic and let be a simply connected, simple algebraic group of type over with Let be the subgroup of type embedded in the usual way, as the stabilizer of a non-singular subspace of the natural -dimensional module for -dimensional Fix a maximal torus of and a maximal torus of such that and let denote Borel subgroups of respectively, with , Let be the corresponding base for the root system of and denote by the corresponding set of fundamental weights for where the natural has highest weight -module Let be a graph automorphism of stabilizing and with the group of , points. Finally, let -fixed be the base for the corresponding root system of associated with the choice of Borel subgroup and denote by , the associated set of fundamental dominant weights for
3.1. Preliminary considerations
For as above and for a -module let denote the vector space equipped with the -action for Clearly is irreducible if and only if is.
Let be a Chevalley basis for the Lie algebra of compatible with our choice of as in Section 2.2. As in Reference 23, Section 8, we may assume that the subalgebra -type of is generated by the root vectors
In particular, we get that for while so that for by Reference 14, Section 13.2, p. 69. Also for write where we set for and by convention. In a similar fashion, for we set
where again we adopt the convention Finally, for we set
where , for every We will require the following relations in
In the remainder of this section, we let be a non-trivial, irreducible having -module highest weight -restricted and fix a maximal vector in for Setting one observes that is a maximal vector of weight in for since The following result provides a necessary and sufficient condition for to be irreducible in the case where
In view of Lemma 3.3, a necessary condition for to act irreducibly on with , such that is for , to belong to for every We conclude this section by showing that is irreducible if and only if for every (see Proposition 3.5 below). We first need the following preliminary lemma.
We now establish the following necessary and sufficient condition for to be generated by as a (and hence for -module to be irreducible by Lemma 3.3), where has highest weight with
3.2. Conclusion
Let be an irreducible having -module non-zero highest weight -restricted and set In this section, we will complete the proof of Theorem 1.2. We first show that for certain weights it is straightforward to see that is reducible. Although the proof of the following proposition can be found in Reference 23, Section 8, we include it here for completeness.
In view of Proposition 3.6, we may and shall assume with throughout the rest of the section, as well as the existence of maximal such that For set and for set
For any sequence write By Lemma 2.1, we have that for every the weight space is spanned by the vectors
where is a maximal vector of weight for We set
The following special case of Reference 8, Theorem A.7, inspired by Reference 13, Proposition 3.1, shall play a key role in the proof of Theorem 1.2.
For we let denote the of -span and The proof of the main result of this section (namely, Theorem 3.9) relies on the following preliminary result.
In the next result, we show that in order to determine whether is irreducible or not, it is enough to determine whether or not, this for every
We are now able to complete the proof of Theorem 1.2.
4. Proof of Propositions 1.5 and 1.7
We first prove Proposition 1.5. Let be a simply connected, simple algebraic group of type over with Let be a semisimple, connected, proper, closed subgroup of Fix a maximal torus of and let denote a Borel subgroup of Also let be the corresponding set of fundamental dominant weights for In this section, we give a proof of Proposition 1.5, starting with three results, proven in a more general setting in Reference 23, Section 5. For the convenience of the reader, and in order to render the current manuscript more self-contained, we include the proofs of these special cases here.
Propositions 4.1 and 4.2 show that under the hypotheses of Proposition 1.5, either Proposition 1.5(a) holds, or acts irreducibly on the natural -module or , is as in Proposition 4.1(ii) with Theorem .1.2 handles the latter situation in case The resolution of this case will follow from induction; see the end of this section. Now for the case where . acts irreducibly on we first show that , must act tensor indecomposably.
For the proof of Proposition 1.5, we continue with our consideration of the case where acts irreducibly and, by the previous result, tensor-indecomposably on the natural -module In particular, we may now assume that . is a simple, proper, closed subgroup of The hypotheses of Proposition .1.5 then imply that is of type or of type Moreover, we have the full set of hypotheses on the embedding of a parabolic subgroup of . in a parabolic subgroup of in particular, with respect to the restriction of the highest weight , to a Levi factor. We will in fact show that the irreducibility of and the hypotheses of Proposition 1.5 are incompatible.
Taking a simple counterexample of minimal rank, we see that we may assume is a maximal parabolic subgroup of In each case, we will require some detailed information about the commutator series of an irreducible . with respect to a fixed maximal parabolic subgroup. We start by considering: -module
Case 1: of type ,
Fix a maximal torus of Let . be a base of the root system of , the corresponding Borel subgroup containing and , a set of fundamental dominant weights chosen with respect to the fixed base Set . to be the reflection corresponding to the root for For a torus of write , for the group of rational characters of Let . be a maximal parabolic subgroup of corresponding to the subset and containing the opposite Borel subgroup , Let . and for a Levi factor of Let be a dominant weight in -restricted set , and assume preserves a non-degenerate quadratic form on and let us denote this by , and the associated bilinear form by .
For a unipotent group and a -module we recall the standard notation , for the subspace spanned by the set of vectors where , and We introduce an additional notation: set . and set for so , .
In case is even, we will consider a certain at level -weight namely the weight ,
Note that
Case 2: .
We conclude this section by showing that the one fixed-rank example acting irreducibly on has no impact on the inductive proof of the main result of Reference 23.
5. Proof of Theorem 1.8
Let be a simply connected, simple algebraic group of type , defined over , We start by considering a maximal, closed, connected semisimple subgroup of satisfying the hypotheses of Theorem ,1.8, namely, has a proper parabolic subgroup whose Levi factor is of type for some Referring to .Reference 18, Theorem 1, we see that we must consider the following pairs :
- •
,
- •
,
- •
.
We start by dealing with the two latter possibilities.
The remainder of this section is devoted to the proof of the following result, first proven by Testerman Reference 26, Theorem 5.0 (i) under the assumption that Reference 23, Table formed a complete family of irreducible triples for the usual embedding .
The proof relies on some detailed knowledge of the structure of certain Weyl modules for and Section 5.1 below provides some results based on the Jantzen formula. In Section -sum5.2, we apply the methods from Section 5.1 to various Weyl modules in order to obtain insight on their structure, as well as information on certain weight multiplicities in the corresponding irreducible quotient. These results shall then prove useful in Section 5.3, in which we conclude by showing that Theorem 5.2 holds. Finally, at the end of the section, we see how these results lead to a proof of Theorem 1.8.
5.1. Understanding Weyl modules
Let be a semisimple algebraic group over and let be a Borel subgroup of containing a fixed maximal torus Let denote the corresponding base for the root system of and let denote the corresponding fundamental dominant weights for Let denote the half-sum of all positive roots in or equivalently, the sum of all fundamental dominant weights. Also for such that define
The following corollary to the strong linkage principle Reference 1 gives a necessary condition for to afford the highest weight of a factor of -composition in the case where and is not of type We refer the reader to Reference 23, 6.2 for a proof.
Let denote the standard basis of the group ring over The Weyl group of acts on by and we write to denote the set of fixed points. The formal character of a given -module is the linear polynomial defined by
Also, for we write
(see Reference 16, II, 2.13, for instance). The Jantzen formula -sumReference 16, II, Proposition 8.19 yields the existence of a filtration of such that and such that the sum denoted satisfies certain properties (loc. cit.). Throughout this section, we call such a filtration the Jantzen filtration of Moreover, since forms a of -basis (see Reference 16, II, Remark 5.8), there exists such that
Consider a -weight with Following Reference 7, Section 3.2, we now define a “truncated” version of which shall prove useful in computations, by setting
where the are as in Equation 5, and where for every we have Finally, the latter decomposition yields
for some The following proposition shows how Equation 7 can be used in order to determine the possible composition factors of together with an upper bound for their multiplicity. We refer the reader to Reference 7, Proposition 3.6 for a proof.
For we call the coefficient in Equation 6 the contribution of to and we say that contributes to if its contribution is non-zero. Now applying Proposition 5.4 for specific weights with requires the knowledge of the contribution of to for each dominant -weight In certain cases, knowing whether or not a given contributes to -weight can be easily determined, as the following result shows. We refer the reader to Reference 7, Lemma 3.7 for a proof.
Fix and recall from Reference 3, Planches I–IV the description of the simple roots and fundamental dominant weights for in terms of a basis for a Euclidean space of dimension For and such that we write , as well as following the ideas of Reference 21. Also following Reference 7, Section 3.2, we set and The action of the Weyl group of on the basis described in Reference 3, Planches I–IV, extends to an action of on in the obvious way. (We write for In addition, define the support of an element ) to be the subset of consisting of those simple roots such that in the decomposition Finally, for we write for the determinant of as an invertible linear transformation of The following result is our main tool for determining the contribution of to for each weight with We refer the reader to Reference 7, Theorem 3.8 for a proof.
5.2. Computing certain weight multiplicities
We now use the results introduced in the previous section to investigate the structure of certain Weyl modules, as well as to compute various weight multiplicities in certain irreducible modules. For and we use the notation for the weight
We will also require some knowledge about the structure of certain Weyl modules for a simple group of type over However, due to the complexity of the description of fundamental dominant weights in terms of an orthonormal basis of a Euclidean space for such it is more convenient to work in a group of type and then deduce the desired result for
5.3. Proof of Theorem 5.2 and conclusion
Let be a simple algebraic group of type over and throughout this section, assume Hypothesis 1.4 for all embeddings with Fix a maximal torus of and let be a Borel subgroup of containing Let denote the corresponding base for the root system of where and denote the set of positive and negative roots, respectively. Also write for the associated fundamental dominant weights. Consider the subgroup of type defined by
where and for Let be the maximal torus of defined by the (see Reference 5, Section 4.3), and let be the Borel subgroup of generated by the and so that is a corresponding base for the root system of (Here again in the obvious way.) We first recall an argument from Reference 26.
We are now ready to give a proof of Theorem 5.2, and to conclude by giving a proof of Theorem 1.8.
Acknowledgments
We would like to thank the anonymous referee whose suggestions and remarks have clarified the exposition. In addition, we thank Tim Burness, Martin Liebeck, Gunter Malle, Gary Seitz, and Jacques Thévenaz for making useful observations on earlier versions of the paper.