THE GROSS-PRASAD CONJECTURE AND LOCAL THETA CORRESPONDENCE. 1. Introduction

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1 THE GROSS-RASAD CONJECTURE AND LOCAL THETA CORRESONDENCE WEE TECK GAN AND ATSUSHI ICHINO 1. Introduction In [15], [16], [9], [1], a restriction problem in the representation theory of classical groups was studied and a precise conjecture was formulated for this restriction problem. This so-called Gross-rasad (G) conjecture has generated much interest in recent years Restriction problem. In this paper, we shall focus on the restriction problem for unitary groups. Thus, let F be a nonarchimedean local field of characteristic and residue characteristic p, and let E be a quadratic field extension of F. Let V n+1 be a Hermitian space of dimension n + 1 over E and W n a skew- Hermitian space of dimension n over E. Let V n V n+1 be a nondegenerate subspace of codimension 1, so that we have a natural inclusion of their corresponding unitary groups U(V n ) U(V n+1 ). In particular, if we set and then we have a diagonal embedding G n = U(V n ) U(V n+1 ) or U(W n ) U(W n ) H n = U(V n ) or U(W n ), : H n G n. Let π be an irreducible smooth representation of G n. determining dim C Hom Hn (π, C). In the Hermitian case, one is interested in We shall call this the Bessel case (B) of the G conjecture. In the skew-hermitian case, the restriction problem requires another piece of data: a Weil representation ω ψ,χ,wn, where ψ is a nontrivial additive character of F and χ is a character of E whose restriction to F is the quadratic character ω E/F associated to E/F by local class field theory. Then one is interested in determining dim C Hom Hn (π, ω ψ,χ,wn ). We shall call this the Fourier-Jacobi case (FJ) of the G conjecture. To unify notation, we shall let ν = C or ω ψ,χ,wn in the respective cases. By surprisingly recent results of Aizenbud-Gourevitch-Rallis-Schiffmann [1] and Sun [34], it is known that the above Hom spaces have dimension at most 1. Thus the main issue is to determine when the Hom space is nonzero. In [9], an answer for this issue is formulated in the framework of the local Langlands correspondence, in its enhanced form due to Vogan [36] which takes into account all pure inner forms. Date: October 9,

2 2 WEE TECK GAN AND ATSUSHI ICHINO 1.2. Local Langlands correspondence. More precisely, a pure inner form of U(V n ) is simply a group of the form U(V n), where V n is a Hermitian space of dimension n over E; likewise in the skew-hermitian case. Thus, a pure inner form of G n is a group of the form G n = U(V n) U(V n+1) or U(W n) U(W n ). We say that such a pure inner form is relevant if V n V n+1 or W n = W n, and V n+1/v n = V n+1 /V n in the Hermitian case. If G n is relevant, we set so that we have a diagonal embedding H n = U(V n) or U(W n), : H n G n. Now suppose that φ is an L-parameter for the group G n. Then φ gives rise to a Vogan L-packet Π φ consisting of certain irreducible smooth representations of G n and its (not necessarily relevant) pure inner forms G n. Moreover, after fixing a Whittaker datum for G n, there is a natural bijection Π φ Irr(S φ ), where S φ is the component group associated to φ. Thus an irreducible smooth representation of G n is labelled by a pair (φ, η), where φ is an L-parameter for G n and η is an irreducible character of S φ. By the recent work of Arthur [2] and Mok [28], the local Langlands correspondence is now known for quasi-split unitary groups, conditional on the stabilization of the twisted trace formula. One can be optimistic that the case of non-quasi-split unitary groups will follow in the near future Gross-rasad conjecture. With this short preparation, the G conjecture can be loosely stated as follows: Gross-rasad conjecture. (i) Given a generic L-parameter φ for G n, there is a unique representation π(φ, η) in the Vogan L-packet Π φ such that π(φ, η) is a representation of a relevant pure inner form G n and such that Hom H n (π(φ, η), ν). (ii) There is a precise recipe for the distinguished character η (which we will recall in 3.2 below). In a stunning series of papers [38], [39], [4], [41], Waldspurger has established the Bessel case of the G conjecture for special orthogonal groups in the case of tempered L-parameters; the case of general generic L-parameters is then dealt with by Mœglin-Waldspurger [27]. Beuzart-lessis [3], [4], [5] has since extended Waldspurger s techniques to settle the Bessel case of the G conjecture for unitary groups in the tempered case urpose of this paper. The purpose of this paper is to establish the Fourier-Jacobi case of the G conjecture, as well as two conjectures of D. rasad concerning local theta correspondence in the (almost) equal rank case, under the following assumptions: We assume the local Langlands correspondence and Arthur s multiplicity formula for unitary groups (see 2.5, 6.5, and (6.3) for the precise statements which will be used). For quasi-split unitary groups, this was proved by Mok [28], under the assumption of the stabilization of the twisted trace formula.

3 THE GROSS-RASAD CONJECTURE AND LOCAL THETA CORRESONDENCE 3 We use the work of Beuzart-lessis [3], [4], [5] on the Bessel case of the G conjecture. This also relies on the local Langlands correspondence for unitary groups (see [5, 7.2]). We further assume that p 2 throughout this paper. This assumption is necessary in order to use the Howe duality, which was proved by Waldspurger [37] for p 2 but is not known for p = 2. However, our main result holds also for p = 2 if we admit the Howe duality. Let us describe the main idea of the proof. As has been noted in [8], the Bessel and Fourier-Jacobi cases of the G conjecture are related by the local theta correspondence. More precisely, there is a see-saw diagram U(W n ) U(W n ) U(V n+1 ) and the associated see-saw identity reads: U(W n ) U(V n ) U(V 1 ) Hom U(Wn)(Θ ψ,χ,vn,w n (σ) ω ψ,χ,v1,w n, π) = Hom U(Vn)(Θ ψ,χ,vn+1,w n (π), σ) for irreducible smooth representations π of U(W n ) and σ of U(V n ). Hence the left-hand side of the see-saw identity concerns the Fourier-Jacobi case (FJ) whereas the right-hand side concerns the Bessel case (B). It is thus apparent that precise knowledge of the local theta correspondence for unitary groups of (almost) equal rank will give the precise relation of (FJ) to (B). More precisely, one would need to know: (Θ) For irreducible tempered representations π and σ, the big theta lifts Θ ψ,χ,vn+1,w n (π) and Θ ψ,χ,vn,w n (σ) are irreducible (if nonzero). (1) If σ has parameter (φ, η) and Θ ψ,χ,vn,w n (σ) has parameter (φ, η ), then (φ, η ) can be precisely described in terms of (φ, η). (2) Likewise, if π has parameter (φ, η) and Θ ψ,χ,vn+1,w n (π) has parameter (φ, η ), then (φ, η ) can be precisely described in terms of (φ, η). In fact, in [3], [31], D. rasad has formulated precise conjectures regarding (1) and (2) for the theta correspondence for U(V n ) U(W n ) and U(V n+1 ) U(W n ) respectively; we shall recall his conjectures precisely in 4. We shall also denote by (weak 1) the part of the conjecture (1) concerning only the correspondence of L-parameters φ φ ; likewise we have (weak 2). Then we recall that in our earlier paper [11], we have shown: roposition 1.1. The statements (Θ), (weak 1) and (weak 2) hold. Using roposition 1.1, the first observation of this paper is: roposition 1.2. Assume (B) and (2). Then (FJ) and (1) follow. In view of roposition 1.2 and the work of Beuzart-lessis [3], [4], [5], it remains to show the statement (2), and our main result is: Theorem 1.3. The conjecture (2), and hence (FJ) and (1), holds. It is interesting to note that in roposition 1.2, the roles of (1) and (2) can be switched. In other words, it is also sufficient to prove (1) in order to prove (FJ). We shall explain in the next subsection why we prefer to prove (2).

4 4 WEE TECK GAN AND ATSUSHI ICHINO 1.5. rasad s conjectures. Given roposition 1.1, the main work is to determine how η depends on (φ, η) in (1) and (2). In fact, the precise determination of η in (1) is a very subtle issue, as it depends on certain local roots numbers. In the case of (2), the dependence of η on (φ, η) is more simplistic. The proof of (2) proceeds by the following steps: First, by our results in [11], the non-tempered case can be reduced to the tempered case on smaller unitary groups. Next, we show that the tempered case can be reduced to the square-integrable case on smaller unitary groups. This is achieved by a nontrivial extension of the techniques in the hd thesis of the second author [2]. Finally, we show the square-integrable case by a global argument. More precisely, we shall globalize an irreducible square-integrable representation π of U(W n ) to an irreducible cuspidal automorphic representation Π = v Π v such that Π v is not square-integrable for all places outside the place of interest, so that (2) is known for Π v outside the place of interest, Π has tempered A-parameter whose global component group is equal to the local component group of the L-parameter of π, Π has nonzero global theta lift to a unitary group which globalizes U(V n+1 ). The desired result then follows for the place of interest by applying Arthur s multiplicity formula for the automorphic discrete spectrum, which can be viewed as a sort of product formula birds and 2 stones. To summarise, in proving our main theorem, we have killed 3 birds (i.e. (FJ), (1) and (2)) with 2 stones (i.e. (B) and Arthur s multiplicity formula), though it is probably more accurate to describe the latter as 2 cannon balls. We can now explain why we prefer to prove (2) rather than (1). Note that one could attempt to follow the same strategy of proof for the statement (1). However, in the globalization step above, we need to ensure that Π has nonzero global theta lift to a certain unitary group. For the case of (1), the nonvanishing of this global theta lift is controlled by the nonvanishing of L( 1 2, Π), and it is well-known that the nonvanishing of this central critical value is a very subtle issue with arithmetic implications. On the other hand, for the statement (2), the nonvanishing of the global theta lift of Π is governed by the nonvanishing of L(1, Π). Now it is certainly much easier to ensure the nonvanishing of L(1, Π) compared to L( 1 2, Π). For example, if Π has tempered A-parameter, then one knows that L(1, Π). It is for this reason that we prove (2) rather than (1). Finally, we mention a related result of Mœglin [26] about the local theta correspondence for symplecticorthogonal dual pairs of arbitrary rank. She considered A-packets for a large class of A-parameters, including all tempered L-parameters, and then determined the analog of the correspondence (φ, η) (φ, η ) in the sense of Arthur, assuming that the correspondence is known for supercuspidal (and slightly more general) representations. Acknowledgements. Notation. Let F be a nonarchimedean local field of characteristic and residue characteristic p. Let F be the normalized absolute value on F. We fix a nontrivial additive character ψ of F. Let E be a quadratic field extension of F and ω E/F the quadratic character of F associated to E/F by local class field theory. Let c denote the nontrivial Galois automorphism of E over F. Let Tr E/F and N E/F be the trace and norm maps from E to F. We choose an element δ E such that Tr E/F (δ) =.

5 THE GROSS-RASAD CONJECTURE AND LOCAL THETA CORRESONDENCE 5 We write = E for the normalized absolute value on E. Let ψ E be the nontrivial additive character of E defined by ψ E = ψ Tr E/F. If G is a linear algebraic group over F, we identify G with its group of F -rational points G(F ). For any totally disconnected locally compact group G, let 1 G be the trivial representation of G and Irr(G) the set of equivalence classes of irreducible smooth representations of G. For any set X, let 1 X be the identity map of X. 2. Local Langlands correspondence We assume the local Langlands correspondence for unitary groups throughout this paper. section, we summarize some of its properties which are used in this paper. In this 2.1. Hermitian and skew-hermitian spaces. Fix ε = ±1. Let V be a finite dimensional vector space over E equipped with a nondegenerate ε-hermitian c-sesquilinear form, V : V V E. Thus we have av, bw V = ab c v, w V, w, v V = ε v, w c V for v, w V and a, b E. ut n = dim V and disc V = ( 1) (n 1)n/2 det V, so that { F /N disc V E/F (E ) if ε = +1; δ n F /N E/F (E ) if ε = 1. We define ɛ(v ) = ±1 by ɛ(v ) = { ω E/F (disc V ) if ε = +1; ω E/F (δ n disc V ) if ε = 1. Given a positive integer n, there are precisely two isometry classes of n-dimensional ε-hermitian spaces V, which are distinguished from each other by their signs ɛ(v ). Note that ɛ(v ) depends on the choice of δ if ε = 1 and n is odd. Let U(V ) be the unitary group of V, i.e. the connected reductive linear algebraic group over F defined by U(V ) = {g GL(V ) gv, gw V = v, w V for v, w V }. If n =, we interpret U(V ) as the trivial group {1} L-parameters and component groups. Let W E be the Weil group of E and WD E = W E SL 2 (C) the Weil-Deligne group of E. We say that a continuous homomorphism φ : WD E GL n (C) is a representation of WD E if φ is semisimple, the restriction of φ to SL 2 (C) is algebraic. We say that φ is tempered if the image of W E is bounded. Let φ be the contragredient representation of φ defined by φ (w) = t φ(w) 1. Fix s W F W E and define a representation φ c of WD E by φ c (w) = φ(sws 1 ). Then the equivalence class of φ c is independent of the choice of s. We say that φ is conjugate self-dual if there is a nondegenerate bilinear form B : C n C n C which satisfies B(φ(w)x, φ c (w)y) = B(x, y) for all w WD E and x, y C n. Namely, φ is conjugate self-dual if and only if φ c is equivalent to φ. For b = ±1, we say that φ is conjugate self-dual with sign b if there is a nondegenerate bilinear form B : C n C n C which satisfies the above condition and the condition that B(y, x) = b B(x, φ(s 2 )y)

6 6 WEE TECK GAN AND ATSUSHI ICHINO for all w WD E and x, y C n. Note that the sign b depends not only on φ but also on B. If φ is conjugate self-dual with sign b (with respect to a bilinear form B), then det φ is conjugate self-dual with sign b n. By [9, 8], an L-parameter for the unitary group U(V ) is an n-dimensional conjugate self-dual representation φ of WD E with sign ( 1) n 1. We may decompose φ into a direct sum φ = i m i φ i with pairwise inequivalent irreducible representations φ i of WD E and multiplicities m i. We say that φ is square-integrable if it is multiplicity-free (so that m i = 1 for all i) and φ i is conjugate self-dual with sign ( 1) n 1 for all i. For an L-parameter φ for U(V ), fix a bilinear form B as above and let Aut(φ, B) be the group of elements in GL n (C) which centralize the image of φ and preserve B. Let S φ = Aut(φ, B)/ Aut(φ, B) be the component group of φ, where Aut(φ, B) is the identity component of Aut(φ, B). As shown in [9, 8], S φ has an explicit description of the form S φ = j (Z/2Z)a j with a canonical basis {a j }, where the product ranges over all j such that φ j is conjugate self-dual with sign ( 1) n 1. In particular, S φ is an elementary abelian 2-group. We shall let z φ denote the image of 1 GL n (C) in S φ. More explicitly, we have z φ = (m j a j ) j (Z/2Z)a j Local Langlands correspondence. The local Langlands correspondence for general linear groups, which was established by Harris-Taylor [18] and Henniart [19], is a certain bijection between Irr(GL n (E)) and equivalence classes of n-dimensional representations of WD E. This bijection satisfies natural properties which determine it uniquely. For example, if π is an irreducible smooth representation of GL n (E) with central character ω π and φ is the n-dimensional representation of WD E associated to π, then ω π = det φ, π is essentially square-integrable if and only if φ is irreducible, π is tempered if and only if φ is tempered. The local Langlands correspondence (as enhanced by Vogan [36]) for unitary groups says that there is a canonical partition Irr(U(V + )) Irr(U(V )) = φ where V + and V are the n-dimensional ε-hermitian spaces with ɛ(v + ) = +1 and ɛ(v ) = 1, the disjoint union on the right-hand side runs over all equivalence classes of L-parameters φ for U(V ± ), and Π φ is a finite set of representations known as a Vogan L-packet. We may decompose Π φ as Π φ = Π + φ Π φ, where for ɛ = ±1, Π ɛ φ consists of the representations of U(V ɛ ) in Π φ. Π φ,

7 THE GROSS-RASAD CONJECTURE AND LOCAL THETA CORRESONDENCE Whittaker data. To describe the L-packet Π φ more precisely, it is necessary to choose a Whittaker datum, which is a conjugacy class of pairs (N, ψ N ), where N is the unipotent radical of a Borel subgroup of the quasi-split unitary group U(V + ), ψ N is a generic character of N. Then relative to this datum, there is a canonical bijection J ψn : Π φ Irr(S φ ). When n is odd, such a datum is canonical. When n is even, as explained in [9, 12], it is determined by the choice of an N E/F (E )-orbit of nontrivial additive characters { ψ E : E/F C if ε = +1; ψ : F C if ε = 1. According to this choice, we write { J ψ E if ε = +1; J ψ if ε = 1 for J ψn. We formally adopt the same notation when n is odd. Suppose that ε = +1, so that V + and V are Hermitian spaces. Let W + = δ V + be the space V + equipped with the skew-hermitian form δ, V +. Similarly, we define the skew-hermitian space W = δ V. Then for ɛ = ±1, U(V ɛ ) and U(W ɛ ) are physically equal. For a given φ, let J ψ E and J ψ be the above bijections for U(V ± ) and U(W ± ) respectively. One has: if n is even, then if n is odd, then J ψ E = J ψ. J ψ E = J ψ ψ E (x) = ψ( 1 2 Tr E/F (δx)), Having fixed the Whittaker datum (N, ψ N ), we shall write π(φ, η) or simply π(η) for the irreducible smooth representation in Π φ corresponding to η Irr(S φ ) under the bijection J ψn. If φ is tempered, then for any Whittaker datum (N, ψ N ), there is a unique (N, ψ N )-generic representation of U(V + ) in Π φ by [5, Lemme 7.1.1], and the irreducible characters of S φ associated to these generic representations under the bijection J ψn are described as follows: The unique (N, ψ N )-generic representation of U(V + ) in Π φ corresponds to the trivial character of S φ. When n is even, there are precisely two Whittaker datum. If (N, ψ N ) is not conjugate to (N, ψ N), then by [21, 3], the unique (N, ψ N )-generic representation of U(V + ) in Π φ corresponds to the character η of S φ given by η (a j ) = ( 1) dim φ j. The character η has a role even when n is odd. Indeed, if n is odd, we may take V = a V +, i.e. the space V + equipped with the Hermitian form a, V +, where a F N E/F (E ). Then U(V + ) and U(V ) are physically equal. Under this identification, we have Π + φ = Π φ for any φ. Let π = π(φ, η) be a representation of U(V + ) in Π φ. If we regard π as a representation of U(V ) via the above identification, then it has associated character η η. In particular, if φ is tempered, then the unique (N, ψ N )-generic representation of U(V ) in Π φ corresponds to η.

8 8 WEE TECK GAN AND ATSUSHI ICHINO 2.5. roperties of the local Langlands correspondence. We highlight some properties of the local Langlands correspondence which are used in this paper: π(φ, η) is a representation of U(V ɛ ) if and only if η(z φ ) = ɛ. π(φ, η) is square-integrable if and only if φ is square-integrable. π(φ, η) is tempered if and only if φ is tempered. If φ is tempered but not square-integrable, then we can write φ = φ 1 φ (φ c 1), where φ 1 is a k-dimensional irreducible representation of WD E for some positive integer k, φ is a tempered L-parameter for U(V ± ± ), where V are the ε-hermitian spaces of dimension n 2k over E. Note that there is a natural embedding S φ S φ. Let η Irr(S φ ) and put ɛ = η (z φ ). We can write V ɛ = X V ɛ X, where X and X are k-dimensional totally isotropic subspaces of V ɛ such that X X is nondegenerate and orthogonal to V ɛ. Let be the maximal parabolic subgroup of U(V ɛ ) stabilizing X and M its Levi component stabilizing X, so that M = GL(X) U(V ɛ ). Let τ be the irreducible (unitary) square-integrable representation of GL(X) associated to φ 1, and let π = π(φ, η ) be the irreducible tempered representation of U(V ɛ) in Π φ corresponding to η. Then the induced representation Ind U(V ɛ ) (τ π ) has a decomposition Ind U(V ɛ ) (τ π ) = π(φ, η), η (2.1) where the sum ranges over all η Irr(S φ ) such that η Sφ = η. Moreover, if φ 1 is conjugate self-dual, let R(w, τ π ) End U(V ɛ )(Ind U(V ɛ ) (τ π )) be the normalized intertwining operator defined in 7.3 below, where w is the unique nontrivial element in the relative Weyl group for M. Then the restriction of R(w, τ π ) to π(φ, η) is the scalar multiplication by { η(a 1 ) if φ 1 has sign ( 1) n 1 ; 1 if φ 1 has sign ( 1) n. If ɛ = +1, then these properties follow from the definition of η, induction in stages, and the local intertwining relation [28, Theorem 3.4.3]. Moreover, when n is odd, one can deduce the case ɛ = 1 from the case ɛ = +1. In general, we can write φ = φ 1 φ r φ (φ c r) (φ c 1), where for i = 1,..., r, φ i is a k i -dimensional representation of WD E of the form φ i = φ i e i for some tempered representation φ i of WD E and real number e i such that e 1 > > e r >, φ is a tempered L-parameter for U(V ± ± ), where V n 2(k k r ) over E. are the ε-hermitian spaces of dimension

9 THE GROSS-RASAD CONJECTURE AND LOCAL THETA CORRESONDENCE 9 Note that the natural map S φ S φ is an isomorphism. Let η Irr(S φ ) and put ɛ = η(z φ ). We can write V ɛ = X 1 X r V ɛ Xr X1, where X i and Xi are k i -dimensional totally isotropic subspaces of V ɛ such that X i Xi are nondegenerate, mutually orthogonal, and orthogonal to V ɛ. Let be the parabolic subgroup of U(V ɛ ) stabilizing the flag X 1 X 1 X 2 X 1 X r and M its Levi component stabilizing the flag X 1 X 1 X 2 X 1 X r, so that M = GL(X 1 ) GL(X r ) U(V ɛ ). Then π(φ, η) is the unique irreducible quotient of the standard module Ind U(V ɛ ) (τ 1 τ r π ), where for i = 1,..., r, τ i is the irreducible essentially tempered representation of GL(X i ) associated to φ i, and π = π(φ, η ) is the irreducible tempered representation of U(V ɛ) in Π φ corresponding to η := η Sφ Irr(S φ ). If π = π(φ, η), then the contragredient representation π of π has L-parameter φ and associated character η π = η ν, where { ω ν(a j ) = E/F ( 1) dim φ j if n is even; 1 if n is odd. Note that the component groups S φ and S φ are canonically identified. In the case of quasi-split unitary groups, this property follows from a result of Kaletha [21, 4]. 3. Gross-rasad conjecture In this section, we explicate the statement of the Gross-rasad conjecture for unitary groups. particular, we recall the definition of the distinguished character η of the component group. In 3.1. airs of spaces. For ɛ = ±1, let V ɛ n denote the n-dimensional Hermitian space with ɛ(v ɛ n) = ɛ and W ɛ n the n-dimensional skew-hermitian space with ɛ(w ɛ n) = ɛ, so that W ɛ n = δ V ɛ n. For the Gross-rasad conjecture, we consider the pair of spaces: V + n V + n+1 or W + n = W + n. Then the relevant pure inner form (other than itself) is and observe that V n V n+1 or W n = W n V ɛ n+1/v ɛ n = L ( 1) n, where for a F, L a denotes the Hermitian line with form a N E/F. We have the groups and G ɛ n = U(V ɛ n) U(V ɛ n+1) or U(W ɛ n) U(W ɛ n) H ɛ n = U(V ɛ n) or U(W ɛ n),

10 1 WEE TECK GAN AND ATSUSHI ICHINO and the embedding : H ɛ n G ɛ n. We also have the Langlands-Vogan parametrization (depending on the choice of the Whittaker datum) relative to the fixed pair of spaces. For an L-parameter φ = φ φ for G ± n, the component group is: S φ = S φ S φ. In particular, under the local Langlands correspondence, the representation π(η) Π φ is a representation of a relevant pure inner form if and only if and π(η) is a representation of G ɛ n if and only if η(z φ, z φ ) = 1, η(z φ, 1) = η(1, z φ ) = ɛ The distinguished character η. We shall now define a distinguished character η Irr(S φ ) when φ = φ φ. Writing S φ = i (Z/2Z)a i and S φ = j (Z/2Z)b j, we thus need to specify the signs η(a i ) = ±1 and η(b j ) = ±1. We consider the Bessel and Fourier-Jacobi cases separately. Bessel case. We fix a nontrivial character ψ E of E/F which determines the local Langlands correspondence for the even unitary group in G ɛ n = U(V ɛ n) U(V ɛ n+1 ). We set ψe 2 (x) = ψe ( 2x) and define: { η (a i ) = ɛ( 1 2, φ i φ, ψ E 2 ); η (b j ) = ɛ( 1 2, φ φ j, ψe 2 ). Fourier-Jacobi case. In this case, we need to fix a nontrivial character ψ of F and a character χ of E with χ F = ω E/F to specify the Weil representation ν = ω ψ,χ,w ɛ n of U(W ɛ n). The recipe for the distinguished character η of S φ depends on the parity of n = dim W ɛ n. If n is odd, recall that det W + n δ N E/F (E ) and define { η (a i ) = ɛ( 1 2, φ i φ χ 1, ψ E 2 ); η (b j ) = ɛ( 1 2, φ φ j χ 1, ψ E 2 ), where ψ E 2 (x) = ψ(tr E/F (δx)). If n is even, the fixed character ψ is used to fix the local Langlands correspondence for U(W ɛ n). We set { η (a i ) = ɛ( 1 2, φ i φ χ 1, ψ E ); η (b j ) = ɛ( 1 2, φ φ j χ 1, ψ E ), where the ɛ-factors are defined using any nontrivial additive character ψ E of E/F. (The result is independent of this choice.) We refer the reader to [9, 18] for a discussion of the various subtleties in the definition of η or η.

11 THE GROSS-RASAD CONJECTURE AND LOCAL THETA CORRESONDENCE Conjectures (B) and (FJ). Let us formally state the statements (B) n and (FJ) n : (B) n Given a tempered L-parameter φ for G ± n = U(V ± n ) U(V ± n+1 ) and a representation π(η) Π φ of a relevant pure inner form G ɛ n, Hom H ɛ n (π(η), C) η = η. (FJ) n Given a tempered L-parameter φ for G ± n = U(W ± n ) U(W ± n ) and a representation π(η) Π φ of a relevant pure inner form G ɛ n, Hom H ɛ n (π(η), ν) η = η. We shall denote by (B) the collection of statements (B) n for all n, and by (FJ) the collection of statements (FJ) n for all n. We stress that both (B) and (FJ) are considered only for tempered representations in this paper. 4. Local theta correspondence and rasad s conjectures In this section, we explicate the statement of rasad s conjectures on the local theta correspondence for unitary groups of (almost) equal rank Weil representations. Let V be a Hermitian space and W a skew-hermitian space. To consider the theta correspondence for the reductive dual pair U(V ) U(W ), one requires certain additional data: (i) a nontrivial additive character ψ of F ; (ii) a pair of characters χ V and χ W of E such that χ V F = ω dim V E/F and χ W F = ω dim W E/F. One way to fix such a pair is simply to fix a character χ of E such that χ F = ω E/F and then set (iii) a trace zero element δ E. χ V = χ dim V and χ W = χ dim W. To elaborate, the tensor product V W has a natural symplectic form defined by Then there is a natural map v 1 w 1, v 2 w 2 = Tr E/F ( v 1, v 2 V w 1, w 2 W ). U(V ) U(W ) Sp(V W ). One has the metaplectic S 1 -cover Mp(V W ) of Sp(V W ), and the character ψ (together with the form, on V W ) determines a Weil representation ω ψ of Mp(V W ). The data (ψ, χ V, χ W, δ) then allows one to specify a splitting of the metaplectic cover over U(V ) U(W ), as shown in [23], [17]. In fact, it does not depend on the choice of δ. Hence, we have a Weil representation ω ψ,χv,χ W,V,W of U(V ) U(W ). The Weil representation ω ψ,χv,χ W,V,W depends only on the orbit of ψ under N E/F (E ).

12 12 WEE TECK GAN AND ATSUSHI ICHINO 4.2. Local theta correspondence. Given an irreducible smooth representation π of U(W ), the maximal π-isotypic quotient of ω ψ,χv,χ W,V,W is of the form Θ ψ,χv,χ W,V,W (π) π for some smooth representation Θ ψ,χv,χ W,V,W (π) of U(V ) of finite length. By a result of Waldspurger [37], the maximal semisimple quotient θ ψ,χv,χ W,V,W (π) of Θ ψ,χv,χ W,V,W (π) is either zero or irreducible when p 2. If χ V and χ W are clear from the context, we simply write Θ ψ,v,w (π) = Θ ψ,χv,χ W,V,W (π) and θ ψ,v,w (π) = θ ψ,χv,χ W,V,W (π). In this paper, we consider the theta correspondence for U(V ) U(W ) with dim V dim W 1. We will state two conjectures of D. rasad which describe the local theta correspondence in terms of the local Langlands correspondence Equal rank case. We first consider the case dim V = dim W = n. We shall consider the theta correspondence for U(V ɛ n) U(W ɛ n ). The following summarises some results of [11]: Theorem 4.1. Let φ be an L-parameter for U(W ± n ). Then we have: (i) For any fixed π Π ɛ φ, exactly one of Θ ψ,v n +,W (π) or Θ n ɛ ψ,vn,w (π) is nonzero. n ɛ (ii) Θ ψ,v ɛ n,w (π) if and only if n ɛ ɛ( 1 2, φ χ 1 V, ψe 2 ) = ɛ ɛ, where ψ2 E (x) = ψ(tr E/F (δx)). (iii) If Θ ψ,v ɛ n,w (π) is nonzero, then it has L-parameter n ɛ θ(φ) = φ χ 1 V χ W. (iv) The theta correspondence π θ ψ,v ɛ n,w (π) gives a bijection n ɛ Π φ Π θ(φ). (v) If φ is tempered and Θ ψ,v ɛ n,w (π) is nonzero, then Θ n ɛ ψ,v (π) is irreducible. n,w ɛ n ɛ 4.4. Conjecture (1). After the above theorem, the remaining question is to specify the bijection of Vogan L-packets given in (iv). We shall do this using the bijections where J ψ : Π φ Irr(S φ ) and J ψ E : Π θ(φ) Irr(S θ(φ) ), (4.1) ψ E (x) = ψ( 1 2 Tr E/F (δx)). Note that the bijections J ψ and J ψ E are independent of ψ and ψ E when n is odd, but when n is even, they do depend on these additive characters and it is crucial for ψ and ψ E to be related as in (4.1) for what follows to hold. Having fixed the bijections J ψ and J ψ E, we need to describe the bijection Irr(S φ ) Irr(S θ(φ) ) η θ(η) induced by the theta correspondence. Note that the component groups S φ and S θ(φ) are canonically identified, since θ(φ) is simply a twist of φ by a conjugate orthogonal character. Now the first conjecture of rasad states the following.

13 THE GROSS-RASAD CONJECTURE AND LOCAL THETA CORRESONDENCE 13 (1) n Let φ be an L-parameter for U(W ± n ) and let η Irr(S φ ). Suppose that S φ = S θ(φ) = i (Z/2Z)a i. Then, relative to J ψ and J ψ E as above, θ(η)(a i )/η(a i ) = ɛ( 1 2, φ i χ 1 V, ψe 2 ), where ψ E 2 (x) = ψ(tr E/F (δx)). We shall denote by (1) the collection of all statements (1) n for all n. Note that we consider (1) for all L-parameters, and not just tempered ones. However, we note: roposition 4.2. Suppose that (1) k holds for all tempered L-parameters for all k < n. Then (1) k holds for all non-tempered L-parameters for all k n. roof. This follows from the analog of [13, Theorem 8.1(iii)] for unitary groups. Moreover, the following is a corollary of Theorem 4.1(ii): Corollary 4.3. The statement (1) n holds if φ is irreducible Almost equal rank case. Now we consider the case dim V = n + 1 and dim W = n. We shall consider the theta correspondence for U(Vn+1 ɛ ɛ ) U(Wn ). The following summarises some results of [11]: Theorem 4.4. Let φ be an L-parameter for U(W ± n ). Then we have: (i) Suppose that φ does not contain χ V. (a) For any π Π ɛ φ, Θ ψ,vn+1 ɛ,w ɛ (π) is nonzero and has L-parameter n θ(φ) = (φ χ 1 V χ W ) χ W. (b) For each ɛ = ±1, the theta correspondence π θ ψ,v ɛ n+1,w (π) gives a bijection n ɛ Π φ Π ɛ θ(φ). (ii) Suppose that φ contains χ V. (a) For any fixed π Π ɛ φ, exactly one of Θ ψ,v + n+1,w (π) or Θ ɛ n ψ,v n+1 case it has L-parameter θ(φ) = (φ χ 1 V χ W ) χ W. (b) The theta correspondence π θ ψ,v ɛ n+1,w (π) gives a bijection n ɛ Π φ Π θ(φ). (iii) If φ is tempered and Θ ψ,v ɛ n+1,w ɛ n (π) is nonzero, then Θ ψ,v ɛ n+1,w ɛ n,w ɛ n (π) is nonzero, in which (π) is irreducible.

14 14 WEE TECK GAN AND ATSUSHI ICHINO 4.6. Conjecture (2). After the above theorem, it remains to specify the bijections given in (i)(b) and (ii)(b). As in the case of (1), we shall do this using the bijections where J ψ : Π φ Irr(S φ ) and J ψ E : Π θ(φ) Irr(S θ(φ) ), ψ E (x) = ψ( 1 2 Tr E/F (δx)). Note that J ψ is independent of ψ when n is odd, whereas J ψ E Observe that: If φ does not contain χ V, then S θ(φ) = S φ (Z/2Z)a, is independent of ψ E when n is even. where the extra copy of Z/2Z arises from the summand χ W in θ(φ). Thus, for each ɛ, one has a canonical bijection Irr(S φ ) Irr ɛ (S θ(φ) ) η θ(η) induced by the theta correspondence, where Irr ɛ (S θ(φ) ) is the set of irreducible characters η of S θ(φ) such that η (z θ(φ) ) = ɛ. On the other hand, if φ contains χ V, then φ χ 1 V χ W contains χ W, so that Thus, one has a canonical bijection induced by the theta correspondence. S θ(φ) = S φ. Irr(S φ ) Irr(S θ(φ) ) η θ(η) Now we can state the second conjecture of rasad. (2) n Let φ be an L-parameter for U(W ± n ) and let η Irr(S φ ). Fix the bijections J ψ and J ψ E as above. If φ does not contain χ V, then θ(η) is the unique irreducible character in Irr ɛ (S θ(φ) ) such that θ(η) Sφ = η. On the other hand, if φ contains χ V, then θ(η) = η. We shall denote by (2) the collection of all the statements (2) n for all n. Note that we consider (2) for all L-parameters, and not just tempered ones. However, we note: roposition 4.5. Suppose that (2) k holds for all tempered L-parameters for all k < n. Then (2) k holds for all non-tempered L-parameters for all k n. roof. This follows from [11, roposition C.4(ii)].

15 THE GROSS-RASAD CONJECTURE AND LOCAL THETA CORRESONDENCE (B) + (2) = (FJ) + (1) In this section, we shall show that Conjectures (FJ) and (1) follow from Conjectures (B) and (2), together with Theorems 4.1 and 4.4. Suppose that we are given tempered L-parameters φ and φ for U(W ± n ). Let be representations such that We first show that π = π(η ) Π ɛ φ and π = π(η ) Π ɛ φ Hom U(W ɛ n )(π π, ω ψ,χ,w ɛ n ). η η = η. Since the representations involved are unitary (as φ and φ are tempered), if and only if Hom U(W ɛ n )(π π, ω ψ,χ,w ɛ n ) Hom U(W ɛ n )((π ) ω ψ,χ,w ɛ n, π ) See-Saw. Now we consider the see-saw diagram (for an ɛ to be determined soon): U(W ɛ n ) U(W ɛ n ) U(V ɛ n+1 ). U(W ɛ n ) U(V ɛ n) U(L ( 1) n) We shall consider the local theta correspondence for the above see-saw diagram. For this, we need to specify precisely the data used in setting up the theta correspondence. More precisely, for the dual pair ɛ ) U(Wn ), we shall use the characters U(V ɛ n+1 and for the dual pair U(V ɛ n) U(W ɛ n ), we use χ V ɛ n+1 = χ n+( 1)n and χ W ɛ n = χn, χ V ɛ n = χ W ɛ n = χn. Then for the dual pair U(L ( 1) n) U(W ɛ n ), we have no choice but to use χ L( 1) n = χ ( 1)n and χ W ɛ n = χn. In particular, the restriction of ω ψ,χl( 1) n,χ Wn ɛ,l ( 1) n,wn ɛ { ωψ,χ,w ɛ n ω ψ,χ,w ɛ n to U(W ɛ n ) is equal to if n is even; if n is odd. In any case, having fixed these normalizations, we shall suppress them from the notation for simplicity. Because of the above differences for even and odd n, it will now be convenient to treat the even and odd cases separately.

16 16 WEE TECK GAN AND ATSUSHI ICHINO 5.2. Even case. Assume first that n is even. By Theorem 4.1, we may choose σ Irr(U(V ɛ n)) such that Θ ψ,v ɛ n,w ɛ n (σ) = (π ). This uniquely determines ɛ. Moreover, by Theorem 4.1, we know that σ has L-parameter since the L-parameter of (π ) is (φ ). φ σ = (φ ), Taking the representation π on U(W ɛ n ) and the representation σ on U(V ɛ n), the resulting see-saw identity reads: By Theorem 4.4, has L-parameter Hom U(W ɛ n )((π ) ω ψ,χ,w ɛ n, π ) = Hom U(V ɛ n )(Θ ψ,v ɛ n+1,w ɛ n (π ), σ). τ := Θ ψ,v ɛ n+1,w ɛ n (π ) φ τ = (φ χ 1 ) χ n. Recall that we have used the character ψ to fix the local Langlands correspondence for U(Wn ɛ ). The component group S φ is of the form S φ = j (Z/2Z)b j and there is a natural embedding S φ S φτ. Now, by (2), the representation τ has associated character η τ Irr(S φτ ) which satisfies: η τ = η on S φ. On the other hand, by (B), one knows exactly what η τ is. Namely, (B) gives: η τ (b j ) = ɛ( 1 2, φ σ φ j χ 1, ψ E ) = ɛ( 1 2, φ φ j χ 1, ψ E ) = η (b j ), where ψ E is any nontrivial character of E/F. Thus, we deduce that η = η on S φ. Now of course we could reverse the role of π and π in the above argument. Then we conclude that as desired. η η = η 5.3. Odd case. Now suppose that n is odd. Then we use the character ψ E (x) = ψ( 1 2 Tr E/F (δx)) of E/F to specify the local Langlands correspondence for U(Vn+1 ɛ ). By Theorem 4.1, we may choose σ Irr(U(Vn)) ɛ such that Θ ψ,v ɛ n,w (σ) = n ɛ π. This uniquely determines ɛ. Moreover, by Theorem 4.1, we know that σ has L-parameter φ σ = φ. Taking the representation (π ) on U(W ɛ n ) and the representation σ on U(V ɛ n), the resulting see-saw identity reads: Hom U(W ɛ n )(π ω, (π ) ) = Hom ψ,χ,w ɛ U(V ɛ n )(Θ ψ,v ɛ n+1,wn ɛ ((π ) ), σ). n

17 THE GROSS-RASAD CONJECTURE AND LOCAL THETA CORRESONDENCE 17 By Theorem 4.4, has L-parameter τ := Θ ψ,v ɛ n+1,w ɛ n ((π ) ) φ τ = ((φ ) χ) χ n. Now by (2), the representation τ has associated character η τ Irr(S φτ ) satisfying: On the other hand, by (B), we know that Hence, we conclude that η τ = η on S φ = S (φ ) χ. η τ (b j ) = ɛ( 1 2, φ σ (φ j ) χ, ψ E 2) = ɛ( 1 2, φ φ j χ 1, ψ E 2 ) = η (b j ). η = η on S φ. Reversing the role of π and π in the above argument, we conclude that as desired. η η = η 5.4. roof of (FJ). At this point, we have shown that if Hom U(W ɛ n )(π π, ω ψ,χ,w ɛ n ), then η η is equal to the distinguished character η. To complete the proof of (FJ), it remains to show that the above Hom space is nonzero for some ɛ and pair of representations (π, π ) Π ɛ Π φ ɛ. φ This will follow from the above see-saw diagram, Theorems 4.1 and 4.4. Let us illustrate this in the case when n is even; the case when n is odd is similar. Consider the tempered L-parameters φ := (φ χ 1 ) χ n for U(V ± n+1 ) and φ := (φ ) for U(V ± n ). By (B), there is a pair of representations such that By Theorem 4.4, we can find a unique π Π ɛ φ Now the see-saw identity gives In particular, (τ, τ ) Π ɛ φ Πɛ φ Hom U(V ɛ n )(τ, τ ). (which determines ɛ ) such that τ = Θ ψ,v ɛ n+1,w ɛ n (π ). Hom U(V ɛ n )(Θ ψ,v ɛ n+1,w ɛ n (π ), τ ) = Hom U(W ɛ n ) (Θ ψ,v ɛ n,w ɛ n (τ ) ω ψ,χ,w ɛ n, π ). π := Θ ψ,v ɛ n,w ɛ n (τ ) and by Theorem 4.1, it has L-parameter (φ ) = φ. Thus we see that for some (π, π ) Π ɛ Π φ ɛ we have Hom U(W ɛ n ) ((π ) ω ψ,χ,w ɛ, π ) n as desired. This completes the proof of (FJ). φ,

18 18 WEE TECK GAN AND ATSUSHI ICHINO 5.5. roof of (1). Now we come to the proof of (1). In particular, we consider the theta correspondence for U(Vn) ɛ U(Wn ɛ ) relative to the Weil representation ω ψ,χv,χ W,Vn ɛ,w ɛ. Given an L-parameter φ n for U(W n ± ), we would like to explicate the bijection θ : Irr(S φ ) Irr(S θ(φ) ) furnished by Theorem 4.1, with θ(φ) = φ χ 1 V χ W. Here, recall that S φ = S θ(φ) = i (Z/2Z)a i. Since we now have (B), (FJ) and (2) at our disposal, we shall be able to determine θ using the see-saw diagram. More precisely, we start with a tempered L-parameter φ and consider an irreducible tempered representation π = π(η) Π ɛ φ. One knows by Theorem 4.1 that Θ ψ,vn ɛ,w (π) ɛ Πɛ n θ(φ) is a nonzero irreducible tempered representation of U(Vn) ɛ for a unique ɛ. By the analog of [13, Lemma 12.5] for unitary groups, one can find an irreducible tempered representation σ of U(Vn 1 ɛ ) such that Hom U(V ɛ n 1 )(Θ ψ,v ɛ n,w (π), σ). n ɛ By (B), one has θ(η)(a i ) = ɛ( 1 2, φ σ φ i χ 1 V χ W, ψ 2), E where φ σ is the L-parameter of σ. On the other hand, one has the see-saw diagram U(W ɛ n ) U(W ɛ n ) U(V ɛ n). U(W ɛ n ) U(V ɛ n 1 ) U(L ( 1) n 1) We consider the theta correspondence for U(L ( 1) n 1) U(Wn ɛ ) relative to the pair of characters (χ ( 1)n 1, χ W ), so that the theta correspondence for U(Vn 1 ɛ ɛ ) U(Wn ) is with respect to the pair (χ V χ ( 1)n, χ W ). We shall suppress these pairs of characters from the notation in the following. By Theorem 4.4, the representation τ := Θ ψ,v ɛ n 1,W ɛ n (σ) is irreducible and tempered. Moreover, τ has L-parameter (5.1) φ τ = (φ σ χ V χ 1 W χ( 1)n ) χ V χ ( 1)n. It will now be convenient to consider the even and odd cases separately Even case. Assume first that n is even. By the see-saw identity, one has It follows by (FJ) that Hom U(W ɛ )(Θ n ψ,vn 1 ɛ,w ɛ n (σ) ω ψ,χ,w ɛ n, π) = Hom U(W ɛ n )(τ π, ω ψ,χ,w ɛ n ). η(a i ) ω E/F ( 1) dim φ i = η π (a i ) = ɛ( 1 2, φ τ φ i χ 1, ψ E 2 ), where the local root number appearing here is independent of the choice of the additive character of E/F used since dim φ τ = n is even. Hence, by (5.1), one has η(a i ) = ɛ( 1 2, φ σ φ i χ V χ 1 W, ψe 2 ) ɛ( 1 2, φ i χ V, ψ E 2 ) ω E/F ( 1) dim φ i.

19 THE GROSS-RASAD CONJECTURE AND LOCAL THETA CORRESONDENCE 19 Noting that φ i is conjugate symplectic, we may compute: θ(η)(a i )/η(a i ) = ɛ( 1 2, φ i χ V, ψ E 2 ) ω E/F ( 1) dim φ i = ɛ( 1 2, φ i χ V, ψ E 2) = ɛ( 1 2, φ i χ 1 V, ψe 2 ) as desired Odd case. Now suppose that n is odd. By the see-saw identity, one has so that By (FJ), one has Hom U(W ɛ )(τ ω n ψ,χ,w, π), n ɛ Hom U(W ɛ n )(τ π, ω ψ,χ,w ɛ n ). η(a i ) = ɛ( 1 2, φ τ φ i χ 1, ψ E 2 ) = ɛ( 1 2, φ σ φ i χ 1 V χ W, ψ2 E ) ɛ( 1 2, φ i χ 1 V, ψe 2 ), where the second equality follows from (5.1). On the other hand, we have seen that θ(η)(a i ) = ɛ( 1 2, φ σ φ i χ 1 V χ W, ψ 2) E = ɛ( 1 2, φ σ φ i χ 1 V χ W, ψ2 E ), where the second equality follows because dim φ σ = n 1 is even. Hence, we conclude that as desired. θ(η)(a i )/η(a i ) = ɛ( 1 2, φ i χ 1 V, ψe 2 ) We have thus shown Conjecture (1) for tempered L-parameters. For non-tempered L-parameters, (1) follows from the tempered case by roposition 4.2. To summarise, we have shown the following proposition: roposition 5.1. Assume that (B) k and (2) k hold for all tempered L-parameters for all k n. Then (FJ) k and (1) k also hold for all tempered L-parameters for all k n (B) + (1) = (FJ) + (2). Instead of assuming (B) and (2) as we have done above, one may assume (B) and (1). Using the same arguments as above, together with Theorems 4.1 and 4.4, one can then deduce (FJ) and (2). We state this formally as a proposition and leave the details of the proof to the reader. roposition 5.2. Assume that (B) k and (1) k hold for all tempered L-parameters for all k n. Then (FJ) k and (2) k also hold for all tempered L-parameters for all k n. 6. roof of (2) After the previous section, and in view of the results of Beuzart-lessis [3], [4], [5] (who proves (B)), it remains to prove (2) n. We shall prove (2) n by using induction on n.

20 2 WEE TECK GAN AND ATSUSHI ICHINO 6.1. The base cases. For (2), there is nothing to prove. By [17], [1] and [5], we know that (B) 1 and (1) 1 hold. Hence it follows by roposition 5.2 that (2) 1 holds. For (2) 2, the non-tempered case follows from the tempered case by roposition 4.5. To show (2) 2 for tempered L-parameters, it follows by roposition 5.2 that it suffices to show (1) 2 for tempered L- parameters. Now (1) 2 was shown in [1, Theorem 11.2] by a global argument, appealing to the analog of (1) 2 at archimedean places. However, we can also give a purely local proof here. Suppose that φ is a tempered L-parameter for U(W 2 ± ) and we are considering the theta correspondence for U(V2 ɛ ɛ ) U(W2 ) with respect to a pair of characters (χ V, χ W ). If φ is irreducible, then Corollary 4.3 guarantees that (1) 2 holds. Hence we shall assume that φ = φ 1 φ 2 with 1-dimensional characters φ i. If φ 1 or φ 2 is not conjugate symplectic, then S φ is trivial and (1) 2 follows from Theorem 4.1. Thus, we shall further assume that both φ 1 and φ 2 are conjugate symplectic, so that { (Z/2Z)a 1 (Z/2Z)a 2 if φ 1 φ 2 ; S φ = ((Z/2Z)a 1 (Z/2Z)a 2 )/ Z/2Z if φ 1 = φ 2. To unify notation in the two cases, we shall regard Irr(S φ ) as a subset of the irreducible characters of (Z/2Z)a 1 (Z/2Z)a 2 even when φ 1 = φ 2. Let π = π(η) Π ɛ φ. By Theorem 4.1, we know that the theta lift of π to U(V ɛ 2 ) is nonzero for a uniquely determined ɛ given by ɛ = ɛ( 1 2, φ χ 1 V, ψe 2 ) ɛ, and has L-parameter θ(φ) = φ χ 1 V χ W. Set σ = Θ ψ,v ɛ (π) Π ɛ 2,W2 ɛ θ(φ) and let θ(η) Irr(S θ(φ) ) be the irreducible character associated to σ. Then we need to compute θ(η)(a i )/η(a i ). Consider the decomposition and choose a character µ Irr(U(V ɛ 1 Then by (B) 1, one sees that )) such that V2 ɛ = V1 ɛ L 1, Hom U(V ɛ 1 )(σ, µ). (6.1) θ(η)(a i ) = ɛ( 1 2, µ 1 E φ iχ 1 V χ W, ψ 2) E = ɛ( 1 2, µ 1 E φ iχ 1 V χ W, ψ2 E ) ω E/F ( 1), where µ E is the character of E given by µ E (x) = µ(x/x c ). On the other hand, consider the see-saw diagram U(W2 ɛ ɛ ) U(W2 ) U(V 2 ɛ). U(W ɛ 2 ) U(V ɛ 1 ) U(L 1) For a conjugate symplectic character χ of E, we consider the theta correspondences for and U(V ɛ 1 ) U(W ɛ 2 ) with respect to (χ V χ, χ W ) U(L 1 ) U(W ɛ 2 ) with respect to (χ 1, χ W ).

21 THE GROSS-RASAD CONJECTURE AND LOCAL THETA CORRESONDENCE 21 Set τ := Θ ψ,χv χ,χ W,V1 ɛ Then Theorem 4.4 implies that τ has L-parameter Now the see-saw identity then gives,w ɛ 2 (µ) on U(W ɛ 2 ). φ τ = µ E χ V χ 1 W χ χ V χ. Hom U(V ɛ 1 )(σ, µ) = Hom U(W ɛ )(τ ω, π). 2 ψ,χ,w ɛ Since we do not know (FJ) 2 at this point, this nonvanishing does not give us the desired information about η. However, we note that Hom U(W ɛ )(τ ω, π) = Hom 2 ψ,χ,w ɛ U(W ɛ 2 )(π ω, τ ). ψ,χ,w ɛ 2 This allows one to exchange the roles of π and τ in a variant of the above see-saw diagram. More precisely, since φ = φ 1 φ 2 with conjugate symplectic characters φ i, it follows by (2) 1 (which we have shown) that the L-packet Π φ can be constructed via theta lifts from U(V 1 ± ). Namely, if we start with the L-parameter φ := φ 1 1 φ 2χ W for U(V 1 ± ) and consider the theta correspondence for U(V1 ɛ ɛ ) U(W2 ) with respect to the pair (φ 1 2, χ W ), then the theta lifts of Π φ give the L-packet Π φ. In particular, we see that for a unique µ Π ɛ φ π = Θ ψ,φ 1 2,χ W,V1 ɛ,w ɛ (µ ) 2 (which determines ɛ ). Indeed, (2) 1 says that (6.2) ɛ = η π (a 1 ) = η(a 1 ) ω E/F ( 1). Thus, we may consider the see-saw diagram 2 2 U(W ɛ 2 ɛ ɛ ) U(W2 ) U(V2 ), and the theta correspondences for U(W2 ɛ ɛ ) U(V1 ) U(L 1) U(V ɛ 1 ) U(W ɛ 2 ) with respect to (φ 1 2, χ W ) and U(L 1 ) U(W2 ɛ ) with respect to (χ 1, χ W ), so that the theta correspondence for U(V2 ɛ ) U(W2 ɛ ) is with respect to (φ 1 2 χ 1, χ W ). The see-saw identity then reads: In particular, Θ ψ,φ 1 Hom U(W ɛ 2 )(π ω, τ ) = Hom ψ,χ,w ɛ U(V ɛ )(Θ 1 ψ,φ 1 2 χ 1,χ W,V2 ɛ,w ɛ χ 1,χ W,V2 ɛ,w ɛ (τ ), µ ). 2 (τ ) on U(V2 ɛ ). By Theorem 4.1(ii), one deduces that ɛ ɛ = ɛ( 1 2, φ τ φ 2 χ, ψ E 2 ) = ɛ( 1 2, µ 1 E φ 2χ 1 χ W, ψ2 E ) ɛ( 1 2, φ 2χ 1 V, ψe 2 ). V

22 22 WEE TECK GAN AND ATSUSHI ICHINO By (6.1) and (6.2), and noting that ɛ = η(a 1 ) η(a 2 ), we see that η(a 2 ) = θ(η)(a 2 ) ɛ( 1 2, φ 2χ 1 V, ψe 2 ) as desired. It then follows by Theorem 4.1(ii) that as well. η(a 1 ) = θ(η)(a 1 ) ɛ( 1 2, φ 1χ 1 V, ψe 2 ) Thus, we have demonstrated (1) 2, and hence (2) Inductive step. Now we assume that n 3 and (2) k holds for all k < n. roposition 4.5 implies that (2) n holds for all non-tempered L-parameters. We are thus reduced to the case of tempered L- parameters. Then we have the following theorem whose proof will be given in the next two sections: Theorem 6.1. If (2) k holds for all tempered L-parameters for all k < n, then (2) n holds for all tempered but non-square-integrable L-parameters. The proof of this theorem is an elaborate extension of the techniques developed in the hd thesis of the second author [2]. Assuming this theorem for the moment, we are thus reduced to the case of square-integrable L-parameters Square-integrable case. We now consider (2) n for a square-integrable L-parameter φ = φ 1 φ r for U(W n ± ). Thus φ is multiplicity-free and each φ i is an n i -dimensional irreducible conjugate self-dual representation of WD E with sign ( 1) n 1. Recall that the component group S φ is of the form r S φ = (Z/2Z)a i. i=1 We shall first assume that r > 1. Then either r 3 or else r = 2 in which case we may assume that n 1 = dim φ 1 2. Let π = π(η) Π ɛ φ character η Irr(S φ ). We consider the theta correspondence for U(Vn+1 ɛ ɛ be an irreducible square-integrable representation of U(Wn ) with associated ɛ ) U(Wn ) with respect to the data (ψ, χ V, χ W ), and suppose that π := Θ ψ,v ɛ n+1,w ɛ n (π). Then by Theorem 4.4, π = π (η ) Π ɛ θ(φ) is an irreducible tempered representation of U(V ɛ n+1 ) with associated character η Irr(S θ(φ) ). We want to determine η in terms of η. Indeed, recall that there is a natural embedding S φ S θ(φ) and we need to show that η (a i ) = η(a i ). We shall do so by a global argument Globalization. Let us begin the process of globalization which is the most delicate part of the argument. Choose a number field F and a quadratic field extension E of F such that Fix: F is totally complex; E v /F v = E/F for a finite place v of F; all 2-adic places of F are split in E; there is a fixed finite place w of F which is split in E.

23 THE GROSS-RASAD CONJECTURE AND LOCAL THETA CORRESONDENCE 23 a nontrivial additive character Ψ of A/F such that Ψ v = ψ (in its N E/F (E )-orbit); a conjugate symplectic Hecke character χ of A E ; a trace zero element δ E so that the signs of the skew-hermitian spaces W ± n at the place v are defined using δ. Let S be a sufficiently large finite set of inert finite places of F, not containing v, such that for all v / S {v }, either v is split in E or else E v /F v, Ψ v and χ v are all unramified. Moreover, S can be made arbitrarily large. We shall globalize the L-parameter φ to a tempered A-parameter Σ. (i) At v, consider the given irreducible representation φ i of WD E. Since φ i is conjugate self-dual with sign ( 1) n 1, it may not be an L-parameter for U(W ± n i ). Instead, the representation φ i,v := φ i χ n i n v is conjugate self-dual with sign ( 1) n i 1, and thus defines an L-parameter for U(W ± n i ). (ii) At v S, choose a representation φ i,v of WD E which is the multiplicity-free sum of 1-dimensional conjugate self-dual characters with sign ( 1) n 1. As above, φ i,v is conjugate self-dual with sign ( 1) n 1 and thus may not be an L-parameter for U(W ± n i,v), where W ± n i,v are the n i -dimensional skew-hermitian spaces over E v. We set φ i,v := φ i,v χ n i n v, so that φ i,v is an L-parameter for U(W n ± i,v). The local component group S φ i,v of φ i,v is of the form S φ i,v = (Z/2Z) n i and the Vogan L-packet Π φ i,v consists of 2 n i irreducible square-integrable representations of U(W ± n i,v). (iii) We require in addition that, for all v S, φ v := φ 1,v φ r,v is not multiplicity-free, i.e. φ v is not a square-integrable L-parameter for U(W ± n,v). To achieve this, we pick a character µ v contained in φ 1,v and then ensure that µ v is also contained in φ iv,v for some i v 2. It is here that we use the assumption that r > 1. Moreover, we may ensure that for some distinct v, v S if r > 2. (iv) For each v S, there is a natural map (Z/2Z) r = i v i v r (Z/2Z)a i S φv i=1 which sends a i to the image of the element 1 φi,v in S φv. In view of (iii), for #S large enough (indeed, for #S 2), the induced diagonal map (Z/2Z) r v S S φv is injective.