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Register a new userDistinction of some induced representations
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Nadir Matringe
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Let $K/F$ be a quadratic extension of $p$-adic fields, $sigma$ the nontrivial element of the Galois group of $K$ over $F$, and $pi$ a quasi-square-integrable representation of $GL(n,K)$. Denoting by $pi^{vee}$ the smooth contragredient of $pi$, and by $pi^{sigma}$ the representation $picirc sigma$, we show that the representation of $GL(2n, K)$ obtained by normalized parabolic induction of the representation $pi^vee otimes pi^sigma$ is distinguished with respect to $GL(2n,F)$. This is a step towards the classification of distinguished generic representations of general linear groups over $p$-adic fields.
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Let $psi : Gto GL(V)$ and $varphi :G to GL (W)$ be representations of finite group $G$. A linear map $T: Vto W$ is called a morphism from $psi$ to $varphi$ if it satisfys $Tpsi_g= varphi_g T$ for each $gin G$ and let $mathrm{Hom}_G (psi ,varphi)$ denote the set of all morphisms. In this paper, we make full stufy of the subspace $mathrm{Hom}_G(psi, varphi)$. As byproducts, we include the proof of the first orthogonality relation and Schurs orthogonality relation.
We give an explicit construction of test vectors for $T$-equivariant linear functionals on representations $Pi$ of $GL_2$ of a $p$-adic field $F$, where $T$ is a non-split torus. Of particular interest is the case when both the representations are ramified; we completely solve this problem for principal series and Steinberg representations of $GL_2$, as well as for depth zero supercuspidals over $mathbf{Q}_p$. A key ingredient is a theorem of Casselman and Silberger, which allows us to quickly reduce almost all cases to that of the principal series, which can be analyzed directly. Our method shows that the only genuinely difficult cases are the characters of $T$ which occur in the primitive part (or type) of $Pi$ when $Pi$ is supercuspidal. The method to handle the depth zero case is based on modular representation theory, motivated by considerations from Deligne-Lusztig theory and the de Rham cohomology of Deligne-Lusztig-Drinfeld curves. The proof also reveals some interesting features related to the Langlands correspondence in characteristic $p$. We show in particular that the test vector problem has an obstruction in characteristic $p$ beyond the root number criterion of Waldspurger and Tunnell, and exhibits an unexpected dichotomy related to the weights in Serres conjecture and the signs of standard Gauss sums.
If $E/F$ is a quadratic extension $p$-adic fields, we first prove that the $mathrm{SL}_n(F)$-distinguished representations inside a distinguished unitary L-packet of $mathrm{SL}_n(E)$ are precisely those admitting a degenerate Whittaker model with respect to a degenerate character of $N(E)/N(F)$. Then we establish a global analogue of this result. For this, let $E/F$ be a quadratic extension of number fields and let $pi$ be an $mathrm{SL}_n(mathbb{A}_F)$-distinguished square integrable automorphic representation of $mathrm{SL}_n(mathbb{A}_E)$. Let $(sigma,d)$ be the unique pair associated to $pi$, where $sigma$ is a cuspidal representation of $mathrm{GL}_r(mathbb{A}_E)$ with $n=dr$. Using an unfolding argument, we prove that an element of the L-packet of $pi$ is distinguished with respect to $mathrm{SL}_n(mathbb{A}_F)$ if and only if it has a degenerate Whittaker model for a degenerate character $psi$ of type $r^d:=(r,dots,r)$ of $N_n(mathbb{A}_E)$ which is trivial on $N_n(E+mathbb{A}_F)$, where $N_n$ is the group of unipotent upper triangular matrices of $mathrm{SL}_n$. As a first application, under the assumptions that $E/F$ splits at infinity and $r$ is odd, we establish a local-global principle for $mathrm{SL}_n(mathbb{A}_F)$-distinction inside the L-packet of $pi$. As a second application we construct examples of distinguished cuspidal automorphic representations $pi$ of $mathrm{SL}_n(mathbb{A}_E)$ such that the period integral vanishes on some canonical copy of $pi$, and of everywhere locally distinguished representations of $mathrm{SL}_n(mathbb{A}_E)$ such that their L-packets do not contain any distinguished representation.
This is Part IV of a thematic series currently consisting of a monograph and four essays. This essay examines the form of induced representations of locally p-adic Lie groups G which is appropriate for the abelian category of ${mathcal M}_{c}(G)$-admissible representations. In my non-expert manner, I prove the analogue of Jacquets Theorem in this category. The final section consists of observations and questions related to this and other concepts introduced in the course of this series.
Let $E/F$ be a quadratic extension of number fields and let $pi$ be an $mathrm{SL}_n(mathbb{A}_F)$-distinguished cuspidal automorphic representation of $mathrm{SL}_n(mathbb{A}_E)$. Using an unfolding argument, we prove that an element of the $mathrm{L}$-packet of $pi$ is distinguished if and only if it is $psi$-generic for a non-degenerate character $psi$ of $N_n(mathbb{A}_E)$ trivial on $N_n(E+mathbb{A}_F)$, where $N_n$ is the group of unipotent upper triangular matrices of $mathrm{SL}_n$. We then use this result to analyze the non-vanishing of the period integral on different realizations of a distinguished cuspidal automorphic representation of $mathrm{SL}_n(mathbb{A}_E)$ with multiplicity $> 1$, and show that in general some canonical copies of a distinguished representation inside different $mathrm{L}$-packets can have vanishing period. We also construct examples of everywhere locally distinguished representations of $mathrm{SL}_n(mathbb{A}_E)$ the $mathrm{L}$-packets of which do not contain any distinguished representation.
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