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Generic cuspidal representations of 𝑈(2, 1)

  • Santosh Nadimpalli ORCID logo EMAIL logo
From the journal Forum Mathematicum

Abstract

Let 𝐹 be a non-Archimedean local field, and let 𝜎 be a non-trivial Galois involution with fixed field F0. When the residue characteristic of F0 is odd, using the construction of cuspidal representations of classical groups by Stevens, we classify generic cuspidal representations of U(2,1)(F/F0).

MSC 2010: 22E50; 11F70

Award Identifier / Grant number: 639.032.528

Funding statement: The author was supported by the NWO Vidi grant “A Hecke algebra approach to the local Langlands correspondence” (nr. 639.032.528).

Appendix A Appendix: Filtration of Uder induced by lattice sequences

In this section, we fix some representatives for 𝐺-conjugacy classes of self-dual lattice sequences on 𝑉 and describe an(Λ) for nZ. Then we use them to determine Uderan(Λ) for nZ. These calculations are used in showing certain representations are non-generic.

A.1 The unramified case

We begin with the case where F/F0 is unramified. Let Λ1 be the lattice sequence of periodicity 2 and

Λ1(-1)=Λ1(0)=oFe1oFe0oFe-1.

The filtration {an(Λ)nZ} of EndF(V) is given by

(A.1)a2m-1(Λ1)=ϖm(oFoFoFoFoFoFoFoFoF)ganda2m(Λ1)=ϖm(oFoFoFoFoFoFoFoFoF)g

for all mZ. Let Λ2 be a period 2 lattice sequence given by

Λ2(0)=oFe1oFe0pFe-1andΛ2(1)=oFe1pFe0pFe-1.

The filtration {an(Λ2)nZ} is given by

(A.2)a2m(Λ2)=ϖm(oFoFpF-1pFoFoFpFpFoF)ganda2m+1(Λ2)=ϖm(pFoFoFpFpFoFpF2pFpF)g

for all mZ. Let Λ3 be the lattice sequence of period 4 given by

Λ3(-1)=oFe1oFe0oFe-1,Λ3(0)=oFe1oFe0pFe-1,
Λ3(1)=oFe1pFe0pFe-1,Λ3(2)=pFe1pFe0pFe-1.
The filtration {an(Λ3)nZ} on 𝔤 is given by

(A.3)a4m+r(Λ3)={ϖm(oFoFoFpFoFoFpFpFoF)gifr=0,ϖm(pFoFoFpFpFoFpFpFpF)gifr=1,ϖm(pFpFoFpFpFpFpFpFpF)gifr=2,ϖm(pFpFpFpFpFpFpF2pFpF)gifr=3.

Although, there is a lattice sequence, say Λ4, with period 6, we do not need to write it down explicitly. This corresponds to type (A) strata, and in this case, all representations are generic. The filtration {Uderan(Λ1)nZ} is given by

Udera2m-1(Λ1)=Udera2m(Λ1)=Uder(m)

for mZ. The filtration {Uderan(Λ2)nZ} is given by

Udera2m(Λ2)=Uder(m-1)andUdera2m+1(Λ2)=Uder(m).

The filtration Λ3 is given by

Udera4m+r(Λ2)={Uder(m)ifr=0,Uder(m)ifr=1,Uder(m)ifr=2,Uder(m+1)ifr=3.

A.2 The ramified case

Now, assume that F/F0 is a ramified extension, and let Λ1 and Λ2 be the lattice sequences of period 2 given by

Λ1(-1)=Λ(0)=oFe1oFe0oFe-1,
Λ2(0)=oFe1oFe0pFe-1andΛ2(1)=oFe1pFe0pFe-1.
The filtration {an(Λ1)nZ} is similar to the filtration in (A.1). The filtration {an(Λ2)nZ}, in this case, is similar to the filtration in (A.2). We will not require to write the filtrations {an(Λ)nZ} for which P0(Λ) is an Iwahori subgroup of 𝐺. The filtration {Uderan(Λ1)nZ} is given by

Udera2m-1(Λ1)=Udera2m(Λ1)=Uder([m/2])

for all mZ. The filtration {Uderan(Λ2)nZ} is given by

Udera2m-1(Λ2)=Udera2m(Λ2)=Uder([(m-1)/2])

for any mZ.

Acknowledgements

I want to thank Maarten Solleveld for many useful discussions and clarifications during the course of this work. I want to thank Shaun Stevens for answering some questions on his paper, for his interest, and other clarifications. I want to thank Peter Badea for suggesting some useful references. I want to thank Kam-Fai Geo Tam for helpful discussions.

  1. Communicated by: Freydoon Shahidi

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Received: 2020-04-24
Revised: 2021-01-25
Published Online: 2021-03-21
Published in Print: 2021-05-01

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