arXiv subjects

Maarten Solleveld

Publications and source records attributed to Maarten Solleveld.

At least 19 recordsLinked to original sources

arXiv2026

A generic categorical local Langlands correspondence for quasi-split reductive groups

We $\textit{unconditionally}$ prove a $\textit{generic}$ categorical local Langlands conjecture for a large class of quasi-split reductive $p$-adic groups $G$, including all quasi-split classical groups and some non-classical groups. More precisely, we construct a natural fully faithful functor from the stable $\infty$-category of $\textit{generic}$ Bernstein blocks on the automorphic side to the stable $\infty$-category of ind-coherent sheaves on the moduli stack of (arithmetic) $L$-parameters, generalizing earlier work of the first author with Ben-Zvi, Chen and Nadler [BZCHN24] for $\mathrm{GL}_n$. Moreover, for an $\textit{arbitrary}$ quasi-split reductive $p$-adic group $G$, we formulate a classical local Langlands framework under which a classical correspondence can be lifted to an $\infty$-categorical correspondence. Our result builds upon the phenomenal recent work of Zhu [Zhu25] on the unipotent block, as well as structure results such as [Sol22] on the automorphic side and [DHKM25] on the spectral side. In particular, our work establishes [HM26,Conjecture 8.2.1], which implies that the conditional proof of [HM26] for the Fargues-Scholze categorical local Langlands equivalence [FS24] (conditional on the conjectured compatibility of the Fargues-Scholze construction with spectral Eisenstein series) applies as well to a large class of quasi-split reductive $p$-adic groups $G$.

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arXiv2025

Standard modules and intertwining operators for reductive p-adic groups

Consider a reductive group G over a non-archimedean local field. The Galois group Gal(C/Q) acts naturally on the category of smooth complex G-representations. We prove that this action stabilizes the class of standard modules. This generalizes and relies on an analogous result about essentially square-integrable representations. Other important objects in the proof of our main result are intertwining operators between parabolically induced G-representations, and the associated Harish-Chandra \mu-functions. We determine an explicit formula for the \mu-function of any irreducible representation of any Levi subgroup of G.

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arXiv2025

Rationality properties of complex representations of reductive p-adic groups

For a reductive group G over a non-archimedean local field, we compare smooth representations over C with smooth representations over Qbar (an algebraic closure of Q). We show that an elliptic G-representation (in the sense of Arthur) can be realized over Qbar if and only if its central character takes values in Qbar. That applies in particular to all essentially square-integrable G-representations. We also study the action of the automorphism group of C/Q on complex G-representations. We prove that the sets of essentially square-integrable representations and of elliptic representations are stable under Gal(C/Q).

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arXiv2025

Group algebras of reductive $p$-adic groups, their representations and their noncommutative geometry

This is a survey paper about representation theory and noncommutative geometry of reductive p-adic groups G. The main focus points are: 1. The structure of the Hecke algebra H(G), the Harish-Chandra-Schwartz algebra S(G) and the reduced C*-algebra $C_r^* (G)$. 2. The classification of irreducible G-representations in terms of supercuspidal representations. 3. The Hochschild homology and topological K-theory of these algebras. In the final part we prove one new result, namely we compute $K_* (C_r^* (G))$ including torsion elements, in terms of equivariant K-theory of compact tori.

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arXiv2025

Endoscopy for representations of disconnected reductive groups over finite fields

Let G be the group of rational points of a connected reductive group over a finite field. Based on work of Lusztig and Yun, we make the Jordan decomposition for irreducible G-representations canonical. It comes in the form of an equivalence between the category of G-representations with a fixed semisimple parameter s and the category of unipotent representations of an endoscopic group of G, enriched with an equivariant structure with respect to the component group of the centralizer of s. Next we generalize these results, replacing the connected reductive group by a smooth group scheme with reductive neutral component. Again we establish canonical equivalences for both the rational and the geometric series of G-representations, in terms of unipotent representations of endoscopic groups.

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arXiv2025

Intertwining operators for representations of covering groups of reductive $p$-adic groups

Let $G$ be a covering group of a reductive $p$-adic group. We study intertwining operators between parabolically induced representations of $G$ and prove that they satisfy certain adjointness relations. The Harish-Chandra $\mu$-function is defined as a composition of such intertwining operators for opposite parabolic subgroups of $G$. It can be seen as a complex rational function and we give an explicit formula for it in terms of poles and zeros. The adjointness of the intertwining operators is an important ingredient to prove the formula for the $\mu$-function. To locate the poles of $\mu$, we construct a continuous family of Hermitian forms on a family of parabolically induced representations.

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arXiv2025

On depth-zero characters of p-adic groups

We show new properties of the Langlands correspondence for arbitrary tori over local fields. Furthermore, we give a detailed analysis of depth-zero characters of reductive p-adic groups, for groups that may be wildly ramified. We present several different definitions of ``depth-zero'' for characters, and show that these notions are in fact equivalent. These results are useful for proving new cases of local Langlands correspondences, in particular for depth zero representations.

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arXiv2024

Hecke algebras and local Langlands correspondence for non-singular depth-zero representations

Let G be a connected reductive group over a non-archimedean local field. We say that an irreducible depth-zero (complex) G-representation is non-singular if its cuspidal support is non-singular. We establish a Local Langlands Correspondence for all such representations. We obtain it as a specialization from a categorical version: an equivalence between the category of finite-length non-singular depth-zero G-representations and the category of finite-length right modules of a direct sum of twisted affine Hecke algebras constructed from Langlands parameters. We also show that our LLC and our equivalence of categories have several nice properties, for example compatibility with parabolic induction.

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arXiv2023

On submodules of standard modules

Consider a standard representation $\pi_{st}$ of a quasi-split reductive p-adic group G. The generalized injectivity conjecture, posed by Casselman and Shahidi, asserts that any generic irreducible subquotient $\pi$ of $\pi_{st}$ is necessarily a subrepresentation of $\pi_{st}$. We will prove this conjecture, improving on the verification for many groups by Dijols. We study this in a geometric way, motivated by favourable properties of Langlands parameters which are open (which means that the nilpotent element from the L-parameter belongs to an appropriate open orbit). Since we do not want to assume a local Langlands correspondence, we involve similar parameters via reduction to Hecke algebras. It does not suffice to pass from G to an affine Hecke algebra, we further reduce to graded Hecke algebras and from there to algebras defined in terms of certain equivariant perverse sheaves. It is in the geometric setting of graded Hecke algebras from cuspidal local systems on nilpotent orbits that we can finally put the ``open" condition on L-parameters to good use. The closure relations between the involved nilpotent orbits provide useful insights in the internal structure of standard modules, which highlight the representations associated with open L-parameters and in particular those for which the enhancement of the L-parameter is trivial. We show that, in the parametrization of irreducible modules of geometric graded Hecke algebras, generic modules always have ``open L-parameters". This leads to a proof of a version of the generalized injectivity conjecture for graded Hecke algebras of geometric type, which is then transferred to reductive p-adic groups.

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arXiv2023

Hermitian duals and generic representations for affine Hecke algebras

We further develop the abstract representation theory of affine Hecke algebras with arbitrary positive parameters. We establish analogues of several results that are known for reductive p-adic groups. These include: the relation between parabolic induction/restriction and Hermitian duals, Bernstein's second adjointness and generalizations of the Langlands classification. We check that, in the known cases of equivalences between module categories of affine Hecke algebras and Bernstein blocks for reductive p-adic groups, such equivalences preserve Hermitian duality. We also initiate the study of generic representation of affine Hecke algebras. Based on an analysis of the Hecke algebras associated to generic Bernstein blocks for quasi-split reductive p-adic groups, we propose a fitting definition of genericity for modules over affine Hecke algebras. With that notion we prove special cases of the generalized injectivity conjecture, about generic subquotients of standard modules for affine Hecke algebras.

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arXiv2023

On principal series representations of quasi-split reductive p-adic groups

Let G be a quasi-split reductive group over a non-archimedean local field. We establish a local Langlands correspondence for all irreducible smooth complex G-representations in the principal series. The parametrization map is injective, and its image is an explicitly described set of enhanced L-parameters. Our correspondence is determined by the choice of a Whittaker datum for G, and it is canonical given that choice. We show that our parametrization satisfies many expected properties, among others with respect to the enhanced L-parameters of generic representations, temperedness, cuspidal supports and central characters. Along the way we characterize genericity in terms of representations of an affine Hecke algebra.

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arXiv2022

Affine Hecke algebras for classical p-adic groups

We consider four classes of classical groups over a non-archimedean local field F: symplectic, (special) orthogonal, general (s)pin and unitary. These groups need not be quasi-split over F. The main goal of the paper is to obtain a local Langlands correspondence for any group G of this kind, via Hecke algebras. To each Bernstein block Rep(G)^s in the category of smooth complex G-representations, an (extended) affine Hecke algebra H(s) can be associated with the method of Heiermann. On the other hand, to each Bernstein component $\Phi_e (G)^{s^\vee}$ of the space \Phi_e (G) of enhanced L-parameters for G, one can also associate an (extended) affine Hecke algebra, say $H (s^\vee)$. For the supercuspidal representations underlying Rep(G)^s, a local Langlands correspondence is available via endoscopy, due to Moeglin and Arthur. Using that we assign to each Rep(G)^s a unique $\Phi_e (G)^{s^\vee}$. Our main new result is an algebra isomorphism $H(s)^{op} \to H (s^\vee)$, canonical up to inner automorphisms. In combination with earlier work, that provides an injective local Langlands correspondence Irr(G) -> \Phi_e (G) which satisfies Borel's desiderata. This parametrization map is probably surjective as well, but we could not show that in all cases. Our framework is suitable to (re)prove many results about smooth G-representations (not necessarily reducible), and to relate them to the geometry of a space of L-parameters. In particular our Langlands parametrization yields an independent way to classify discrete series G-representations in terms of Jordan blocks and supercuspidal representations of Levi subgroups. We show that it coincides with the classification of the discrete series obtained twenty years ago by Moeglin and Tadi\'c.

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arXiv2022

Graded Hecke algebras and equivariant constructible sheaves on the nilpotent cone

Graded Hecke algebras can be constructed geometrically, with constructible sheaves and equivariant cohomology. The input consists of a complex reductive group G (possibly disconnected) and a cuspidal local system on a nilpotent orbit for a Levi subgroup of G. We prove that every such "geometric" graded Hecke algebra is naturally isomorphic to the endomorphism algebra of a certain G x C*-equivariant semisimple complex of sheaves on the nilpotent cone $g_N$. From there we provide an algebraic description of the G x C*-equivariant bounded derived category of constructible sheaves on $g_N$. Namely, it is equivalent with the bounded derived category of finitely generated differential graded modules of a suitable direct sum of graded Hecke algebras. This can be regarded as a categorification of graded Hecke algebras.

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arXiv2022

Hochschild homology of reductive $p$-adic groups

Consider a reductive $p$-adic group $G$, its (complex-valued) Hecke algebra $H(G)$ and the Harish-Chandra--Schwartz algebra $S(G)$. We compute the Hochschild homology groups of $H(G)$ and of $S(G)$, and we describe the outcomes in several ways. Our main tools are algebraic families of smooth $G$-representations. With those we construct maps from $HH_n (H(G))$ and $HH_n (S(G))$ to modules of differential $n$-forms on affine varieties. For $n = 0$ this provides a description of the cocentres of these algebras in terms of nice linear functions on the Grothendieck group of finite length (tempered) $G$-representations. It is known from earlier work that every Bernstein ideal $H(G)^s$ of $H(G)$ is closely related to a crossed product algebra of the from $O(T) \rtimes W$. Here $O(T)$ denotes the regular functions on the variety $T$ of unramified characters of a Levi subgroup $L$ of $G$, and $W$ is a finite group acting on $T$. We make this relation even stronger by establishing an isomorphism between $HH_* (H(G)^s)$ and $HH_* (O(T) \rtimes W)$, although we have to say that in some cases it is necessary to twist $C[W]$ by a 2-cocycle. Similarly we prove that the Hochschild homology of the two-sided ideal $S(G)^s$ of $S(G)$ is isomorphic to $HH_* (C^\infty (T_u) \rtimes W)$, where $T_u$ denotes the Lie group of unitary unramified characters of $L$. In these pictures of $HH_* (H(G))$ and $HH_* (S(G))$ we also show how the Bernstein centre of $H(G)$ acts. Finally, we derive similar expressions for the (periodic) cyclic homology groups of $H(G)$ and of $S(G)$ and we relate that to topological K-theory.

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arXiv2022

A comparison of Hochschild homology in algebraic and smooth settings

Consider a complex affine variety $\tilde V$ and a real analytic Zariski-dense submanifold V of $\tilde V$. We compare modules over the ring $O (\tilde V)$ of regular functions on $\tilde V$ with modules over the ring $C^\infty (V)$ of smooth complex valued functions on V. Under a mild condition on the tangent spaces, we prove that $C^\infty (V)$ is flat as a module over $O (\tilde V)$. From this we deduce a comparison theorem for the Hochschild homology of finite type algebras over $O (\tilde V)$ and the Hochschild homology of similar algebras over $C^\infty (V)$. We also establish versions of these results for functions on $\tilde V$ (resp. V) that are invariant under the action of a finite group G. As an auxiliary result, we show that $C^\infty (V)$ has finite rank as module over $C^\infty (V)^G$.

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arXiv2022

Hochschild homology of affine Hecke algebras

Let H = H (R,q) be an affine Hecke algebra with complex, possibly unequal parameters q, which are not roots of unity. We compute the Hochschild and the cyclic homology of H. It turns out that these are independent of q and that they admit an easy description in terms of the extended quotient of a torus by a Weyl group, both of which are canonically associated to the root datum R. For q positive we also prove that the representations of the family of algebras H (R,q^ε) come in families which depend analytically on the complex number ε. Analogous results are obtained for graded Hecke algebras and for Schwartz completions of affine Hecke algebras. Correction: in 2021 some problems with the construction of families of representations surfaced. These are discussed in additional comments in Section 2.

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arXiv2022

Hochschild homology of twisted crossed products and twisted graded Hecke algebras

Let A be a \C-algebra with an action of a finite group G, let $\natural$ be a 2-cocycle on $G$ and consider the twisted crossed product $A \rtimes \C [G,\natural]$. We determine the Hochschild homology of $A \rtimes \C [G,\natural]$ for two classes of algebras A: - rings of regular functions on nonsingular affine varieties, - graded Hecke algebras. The results are achieved via algebraic families of (virtual) representations and include a description of the Hochschild homology as module over the centre of $A \rtimes \C [G,\natural]$. This paper prepares for a computation of the Hochschild homology of the Hecke algebra of a reductive p-adic group.

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arXiv2021

Graded Hecke algebras, constructible sheaves and the p-adic Kazhdan--Lusztig conjecture

Graded Hecke algebras can be constructed in terms of equivariant cohomology and constructible sheaves on nilpotent cones. In earlier work, their standard modules and their irreducible modules where realized with such geometric methods. We pursue this setup to study properties of module categories of (twisted) graded Hecke algebras, in particular what happens geometrically upon formal completion with respect to a central character. We prove a version of the Kazhdan--Lusztig conjecture for (twisted) graded Hecke algebras. It expresses the multiplicity of an irreducible module in a standard module as the multiplicity of an equivariant local system in an equivariant perverse sheaf. This is applied to smooth representations of reductive p-adic groups. Under some conditions, we verify the p-adic Kazhdan--Lusztig conjecture from [Vogan]. Here the equivariant constructible sheaves live on certain varieties of Langlands parameters. The involved conditions are checked for substantial classes of groups and representations.

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