Abstract. For a generic L-parameter of U (n) × U (n), it is conjectured that there is a unique representation in their associated relevant Vogan L-packet which produces the unique Fourier-Jacobi model. We investigated this conjecture for some non-generic L-parameters of U (3) × U (3) and discovered that this conjecture is still true for some non-generic L-parameter and false for some non-generic L-parameter. In the case when it holds, we specified such representation under the local Langlands correspondence for unitary group.
IntroductionThe local Gan-Gross-Prasad conjecture deals with certain restriction problems between p-adic groups. In this paper, we shall investigate it for some non-generic case not yet treated before.Let E/F be a quadratic extension of number fields and G = U (3) be the quasi-split unitary group of rank 3 relative to E/F . Then H = U (2) × U (1) is the unique elliptic endoscopic group for G. In [23], Rogawski has defined a certain enlarged class of L-packets, or A-packets, of G using endoscopic transfer of one-dimensional characters of H to G. In more detail, let ̺ = ⊗ v ̺ v be a one-dimensional automorphic character of H. The A-packet Π(̺) ≃ ⊗Π(̺ v ) is the transfer of ̺ with respect to functoriality for an embedding of L-groups ξ : L H → L G. Then for all places v of F , Π(̺ v ) contains a certain non-tempered representation π n (̺ v ) and it contains an additional supercuspidal representaton π s (̺ v ) precisely when v remains prime in E. Gelbart and Rogawski [12] showed that the representations in this A-packet arise in the Weil representation of G. Our goal is to study the branching rule of the representations in this A-packet.For the branching problem, there is a fascinating conjecture, the so-called Gan-Gross-Prasad (GGP) conjecture, which was first proposed by Gross and Prasad [7] for orthogonal group and later they, together with Gan, extended it to all classical group in [6]. Since our main theorem has to do with it, we shall give a brief review on the GGP conjecture, especially for unitary group. Let E/F be a quadratic extension of non-archimedean local fields of characteristic zero. 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 if we set G n = U(V n ) × U(V n+1 ) or U(W n ) × U(W n ) and H n = U(V n ) or U(W n ),