HOLOMORPHIC SPHERES IN LOOP GROUPS AND BOTT PERIODICITY*

In this paper we study the topology of spaces of holomorphic maps from the Riemann sphere P to infinite dimensional Grassmanian manifolds and to loop groups. Included in this study is a complete identification of the homotopy types of i7o/fc(P,S? 7(n)) and of JJoZfc(P,fiC/'), where the subscript k denotes the degree of the map. These spaces are shown to be homotopy equivalent to the k Mitchell Segal algebraic filtration of the loop group QU(n) [7], and to BU(k), respectively. Introduction. One of the most important theorems in Topology and Geometry is the "Bott Periodicity Theorem". In its most basic form it states that there is a natural homotopy equivalence, (3:ZxBU ^—> QU. Here U is the infinite unitary group, U = limnU(n), BU — \minBU{ri) is the limit of the classifying spaces, and QU = C(S,U) is the space of smooth, basepoint preserving loops. Here and throughout the rest of this paper all spaces will assumed to be equipped with a basepoint, and all maps and mapping spaces will be basepoint preserving. If we input the fact that U ~ HBU, Bott periodicitiy states that there is a natural homotopy equivalence piZxBU —■=-> nBU = C{S,BU). In a paper which first pointed to the deep relationship between the index theory of Fredholm operators and Algebraic Topology, Atiyah [1], defined a homotopy inverse to the Bott map /? , which can be viewed as a map d : G{S,BU) ) Z x BU. This map was defined by studying the index of the family of operators obtained by coupling the 8 operator to a smooth map from S to a Grassmannian. Since the mapping spaces C(S,BU) and QU both have path components naturally identified with the integers, we denote by C(S,BU)k the path components consisting of degree k maps. Thus Bott periodicity, together with Atiyah's results says that for each integer &, there is a natural homotopy equivalence, d:C(S,BU)k ^^-> BUThe goal of this paper is to prove a holomorphic version of this result. We first note however that the homotopy type of BU has many different models, several of which carry a holomorphic structure. For the purposes of this paper we think of BU * Received July 30, 1999, accepted for publication January 10, 2000. "''Dept. of Mathematics, Stanford University, Stanford, California 94305, USA (ralph@math. stanford.edu). The research of this author was partially supported by a grant from the NSF. ^Department of Mathematics, University of Michigan, Ann Arbor, Michigan 48109, USA (lupercio @math.lsa.umich.edu). The research of this author received partial support from CONACYT. §Dept. of Pure Mathematics and Mathematical Statistics, Cambridge University, Cambridge, England (G.B.Segal@dpmms.cam.ac.uk). 801 802 R. L. COHEN, E. LUPERCIO, AND G. B. SEGAL as the colimit of the finite dimensional Grassmannians Grm (C ) of m dimensional subspaces of C. We define Hol^^BU) to be lii^m,niJo/fe(P ,Grm(C )) topologized as a subspace of C(S,BU) = lir^ m,nC(S', Grm(C )), where these mapping spaces are given the compact open topology. The following is the first result of this paper. THEOREM 1. For each positive integer k, there is a natural homotopy equivalence Holk(¥\BU) —=-> BU(k). REMARK. Observe that this theorem states implies the inclusion of holomorphic maps into all smooth maps (of degree k), Holk(r\BU) M> C ^,^/)* is homotopy equivalent to the inclusion of classifying spaces, BU(k) <-> BU. In future work we will study consequences of this theorem to the understanding of holomorphic K theory of a smooth, projective variety. Our other main result in this paper has to do with an identification of the topology of subspaces iJo/fc(P,5^7(n)) of iJo/j^P, £? [/)• By theorem 1, this latter space is homotopy equivalent to BU(k), and so the inclusion BU{ri) C BU induces a map j : Holk(r\BU(n)) -+ BU(k). On the other hand, the inclusion of holomorphic maps into smooth maps, together with the identification of ftBU(n) with ClU(n) induces a map i : Holk(T\BU(n)) -+ ilkU(n). Furthermore, it is not difficult to see that the maps j and i are compatible when composed to BU ~ flU. Now S. Mitchell in [7] described an algebraic filtration of the loop group QSU(n) by compact, complex subvarieties: Fhn C F2,n C • • • C FKn C • • • C i^ ~ nSU(n). In this filtration, Fijn = CP n_1 included in ftU(n) via the usual complex J map. Fk,n C nU(ri) is the set of all k fold products of elements of Fi,n C QU(n). See [11]. The subspaces Fk^n have as their homology, precisely the intersection of H*{BU(k)) and H*(QSU(n)), viewed as subgroups o£H*(BU) = H^(VtSU). In [8], Richter proved that this filtration stably splits, so that the loop group ClSU(n) is stably homotopy equivalent to a wedge of the subquotients, Fk+i,n/Fk,n. The following theorem was conjectured by Mann and Milgram in [6] after an analysis of the holomorphic mapping spaces fl'o/ifc(P ,Grm(e )): THEOREM 2. There is a natural homotopy equivalence Holk(F\BU(n))~Fk,n. HOLOMORPHIC SPHERES IN LOOP GROUPS AND BOTT PERIODICITY 803 These theorems are proven by using the identification of the loop group £lU(ri) with a certain moduli space of holomorphic bundles over P, together with holomorphic trivializations on a disk. This identification was established in [9]. We then identify the homotopy type of JH"oZjfe(P, J3l7(n)) with a subspace of this moduli space consisting of bundles that are "negative" in the sense that they are (holomorphically) isomorphic to a direct sum of line bundles, each of which has nonpositive first Chern class. The topology of these moduli spaces are then studied in two ways: homologically, using calcluations of [6], and Morse theoretically, by analyzing the gradient flow of the Dirichlet energy functional on CtU(n). This paper is organized as follows. In section 1 we review some results from the theory of loop groups. The main reference for this material is Pressley and Segal's book [9]. This theory will in particular allow us to define the terms and maps in the statements of the above theorems more carefully. In section 2 we give a proof of theorems 1 and 2, modulo a technical argument establishing that certain maps are (quasi) fibr at ions. This argument is carried out in section 3. The real Bott periodicity analogues of the above theorems (where the unitary groups are replaced by orthogonal and symplectic groups) were established in the Stanford University Ph.D thesis of the second author written under the direction of the first author. The authors are grateful to Paulo Lima-Filho, Steve Mitchell, Paul Norbury, and Giorgio Valli for helpful conversations regarding this work. 1. Loop Groups. In this section we recall some of the basic constructions from the theory of loop groups as developed in [9]. We will use these constructions to define the holomorphic structures necessary to define the spaces and maps in the theorems described in the introduction. In what follows we will work with the Lie groups f7(n), but everything we use has obvious analogues for arbitrary compact semi-simple Lie groups. Again, we refer the reader to [9] for details. As defined in the introduction, let LG denote the space of smooth maps from the circle 5 to a Lie group G. The loop group LGL(n, C) has the following important subgroups. 1. The group LGL(n, C) of maps 7 : 5 ->■ GL(n,C) that extend to holomorphic maps of the closed disk D ->• GL(n,C). 2. the group Lp0iGL(n, C) of loops whose matrix entries are finite Laurent polynomials in z. That is, loops 7 of the form