Colin J Bushnell To an Effective Local Langlands Correspondence

Annotation

Let $F$ be a non-Archimedean local field. Let $\mathcal{W}_{F}$ be the Weil group of $F$ and $\mathcal{P}_{F}$ the wild inertia subgroup of $\mathcal{W}_{F}$. Let $\widehat {\mathcal{W}}_{F}$ be the set of equivalence classes of irreducible smooth representations of $\mathcal{W}_{F}$. Let $\mathcal{A}^{0}_{n}(F)$ denote the set of equivalence classes of irreducible cuspidal representations of $\mathrm{GL}_{n}(F)$ and set $\widehat {\mathrm{GL}}_{F} = \bigcup _{n\ge 1} \mathcal{A}^{0}_{n}(F)$. If $\sigma \in \widehat {\mathcal{W}}_{F}$, let $^{L}{\sigma }\in \widehat {\mathrm{GL}}_{F}$ be the cuspidal representation matched with $\sigma $ by the Langlands Correspondence. If $\sigma $ is totally wildly ramified, in that its restriction to $\mathcal{P}_{F}$ is irreducible, the authors treat $^{L}{\sigma}$ as known. From that starting point, the authors construct an explicit bijection $\mathbb{N}:\widehat {\mathcal{W}}_{F} \to \widehat {\mathrm{GL}}_{F}$, sending $\sigma $ to $^{N}{\sigma}$. The authors compare this "naive correspondence" with the Langlands correspondence and so achieve an effective description of the latter, modulo the totally wildly ramified case. A key tool is a novel operation of "internal twisting" of a suitable representation $\pi$ (of $\mathcal{W}_{F}$ or $\mathrm{GL}_{n}(F)$) by tame characters of a tamely ramified field extension of $F$, canonically associated to $\pi $. The authors show this operation is preserved by the Langlands correspondence.