TWISTED LOOP GROUPS AND THEIR AFFINE FLAG VARIETIES G. PAPPAS* AND M. RAPOPORT Introduction Loop groups are familiar objects in several branches of mathematics. Let us mention here three variants. The first variant is differential-geometric in nature. One starts with a Lie group G (e.g., a compact Lie group or its complexification). The associated loop group is then the group of (C0-, or C1-, or C∞-) maps of S1 into G, cf. [P-S] and the literature cited there. A twisted version arises from an automorphism α of G. The associated twisted loop group is the group of maps γ : R → G such that γ(θ + 2π) = α(γ(θ)) . The second variant is algebraic and arises in the context of Kac-Moody algebras. Here one constructs an infinite-dimensional algebraic group variety with Lie algebra equal or closely related to a given Kac-Moody algebra. (This statement is an oversimplification and the situation is in fact more complicated: there exist various constructions at a formal, a minimal, and a maximal level which produce infinite-dimensional groups with Lie algebras closely related to the given Kac-Moody Lie algebra, see [Ma2], also [T2], [T3] and the literature cited there). The third variant is algebraic-geometric in nature and is our main concern in this paper. Let us recall the basic definitions in the untwisted case. Let k be a field and let G0 be an algebraic group over Spec (k). We consider the functor LG0 on the category of k-algebras, R 7→ LG0(R) = G0(R((t))). Here R((t)) = R[[t]][1/t] is the ring of Laurent series with coefficients in R. One proves that this functor is representable by an ind-scheme (= inductive limit of k-schemes), called the algebraic loop group associated to G0. At the same time, one considers the associated flag varieties, mostly the affine Grassmannian G and the affine flag variety F, the fpqc-sheaves associated to the functors, R 7→ G0(R((t)))/G0(R[[t]]), resp. R 7→ G0(R((t)))/B(R). Date: August 17, 2006. *Partially supported by NSF grants # DMS05-01049 and # DMS01-11298 (via the Institute for Advanced Study). 1 2 G. PAPPAS AND M. RAPOPORT Here B(R) is the inverse image of a Borel subgroup under the reduction map G0(R[[t]]) → G0(R). These sheaves are also representable by ind-schemes. Important results on the structure of algebraic loop groups and their associated flag varieties are due to Beauville, Laszlo, Sorger, Faltings, Beilinson, Drinfeld and Gaitsgory, cf. [B-L], [BLS], [L-S], [Fa1], [B-D], [G]. These results have applications in the theory of vector bundles on algebraic curves [B-L], [L-S], [Fa1], in geometric Langlands theory [B-D], [G], and to local models of Shimura varieties [Go1], [Go2], [P-R]. The aim of the present paper is to develop a similar theory of twisted algebraic loop groups and of their associated flag varieties. As it turns out, this theory gives a geometric interpretation of many aspects of Bruhat-Tits theory in the equal characteristic case. The basic definition is very simple. Let G be a linear algebraic group over K = k((t)) (we always assume that G is connected reductive). Then G defines its associated algebraic loop group LG, which is the ind-scheme representing the functor R 7→ LG(R) = G(R((t))). Note that, since R((t)) is a k((t))-algebra, the right hand side makes sense. When G = G0 ⊗k K, one recovers the definition in the untwisted case. To any facet a in the Bruhat- Tits building of G over K there is associated a unique smooth group scheme with connected fibers Pa over k[[t]] such that Pa(k[[t]]) is equal to the parahoric subgroup of G(K) attached + to a. To Pa corresponds an infinite-dimensional affine group scheme L Pa over Spec (k) with + L Pa(R) = Pa(R[[t]]). + The quotient Fa = LG/L Pa (in the sense of fpqc-sheaves) is representable by an ind- scheme over k which we call the (partial) affine flag variety associated to G and a. In the untwisted case, the affine Grassmannian is associated to the special vertex arising from an isomorphism G ' G0 ⊗k k((t)), and the affine flag variety is associated to an alcove in the Bruhat-Tits building containing this vertex. Note that even in the untwisted case, the definition of Fa is useful, as it re-establishes the natural symmetry attached to the K-group scheme G0 ⊗k K which has no preferred special vertex. Our main results may now be formulated as follows. Let π1(G) denote the algebraic fundamental group of G in the sense of Borovoi. If k is algebraically closed, let I = Gal(K/K) denote the inertia group. Let π1(G)I be the co-invariants under I. Theorem 0.1. Let k be algebraically closed. The Kottwitz homomorphism ([Ko], see 2.a.2) induces bijections on the set of connected components, π0(LG) = π1(G)I , π0(Fa) = π1(G)I . Note that in the untwisted case G = G0 ⊗k K, the action of I is trivial so that π1(G)I = π1(G) and we recover the result of Beauville, Laszlo and Sorger [BLS], Lemma 1.2., resp. of TWISTED LOOP GROUPS 3 Beilinson and Drinfeld [B-D], Prop. 4.5.4, which is analogous to the corresponding result in topology. Theorem 0.2. Suppose that G is semi-simple and splits over a tamely ramified extension of K and that the order of the fundamental group of the derived group π1(Gder) is prime to the characteristic of k. Then the ind-schemes LG and Fa are reduced. In the case of SLn this is due to Beauville and Laszlo [B-L], and in the case of a general split group and when k has characteristic 0, to Beilinson and Drinfeld [B-D], Thm. 4.5.1. In characteristic p > 0, this is due to Laszlo and Sorger [LS], and to Faltings [Fa1], when G is split and simply connected. We next consider the Schubert varieties contained in a partial affine flag variety Fa. By + definition, these are the reduced closures of orbits of L Pa. They are finite-dimensional projective varieties. Theorem 0.3. Suppose that G splits over a tamely ramified extension of K. Then all Schubert varieties in Fa are normal and have only rational singularities. In positive char- acteristic, they are Frobenius-split. In characteristic zero, normality of Schubert varieties in the context of Kac-Moody partial affine flag varieties is due to Kumar [Ku1], Littelmann [Li] and Mathieu [Ma1]. By an argument of Faltings these coincide with the Schubert varieties in this paper. In positive characteristic the above theorem is due to Faltings [Fa1] in the split simply connected case. (The normality of Schubert varieties in the case of SLn is also proved in [P-R1]). Normality of Schubert varieties in characteristic p has also been shown by Mathieu [Ma1] and Littelmann [Li] (in the context of Kac-Moody theory). Their definition of Schubert varieties is a priori different. They are given using an embedding into the infinite dimensional projective space associated to a highest-weight representation of the Kac-Moody algebra. In contrast, we are essentially defining Schubert varieties as varieties of certain lattices and so [Ma1] and [Li] does not imply our result. Nevertheless, it is a corollary of Theorem 0.3 combined with the results of Mathieu and Littelmann that the two notions of Schubert varieties coincide (see Section 9.h). From this, it also follows a posteriori that (for an absolutely simple, simply connected G that splits over a tame extension) our partial affine flag varieties are isomorphic to the ones defined in the Kac-Moody theory. Let us now comment on our proofs of the above theorems. The proofs of theorems 0.2 and 0.3 are closely intertwined. The proofs go along the lines of the proof of the theorem corresponding to Theorem 0.3 of Faltings [Fa1] (for more details on this proof, cf. [Go2]). The main method is to lift the situation in positive characteristic to characteristic zero and compare the lifted situation with the Kac-Moody setting. It is therefore somewhat reminiscent of the method introduced by Kazhdan to relate the representation theory of a group over a local field of characteristic p to that of a group over a local field of unequal characteristic. This also explains why we have to exclude groups which split over a wild 4 G. PAPPAS AND M. RAPOPORT extension (although we strongly believe that appropriate versions of Theorems 0.2 and 0.3 also hold in this case). We note that in the previous proofs of theorem 0.2 (in [BL] for the case of SLn, in [LS] for a split semi-simple, simply connected group in characteristic 0, in [Fa1, Go2] for a semi-simple split simply connected group in characteristic p) all use the big cell. Our argument, which we learned in Faltings’ course [Fa2] is different. Finally, the proof of Theorem 0.1 generalizes the proof of the corresponding theorem in [B-D], using the construction of the Kottwitz map [Ko]. We would like to stress that interesting problems arise when one tries to generalize to the setting of twisted loop groups classical results in the untwisted case. The theory of perverse sheaves on their partial flag varieties has still to be worked out. Also, we expect a theory of bundles on algebraic curves for corresponding non-constant algebraic groups, of their Picard groups and their theta functions.