Pith. sign in

REVIEW 2 major objections 3 minor 34 references

Stability of Elliptic Fargues-Scholze $L$-packets

T0 review · 2 major / 3 minor · reviewed 2026-08-10 · deepseek-v4-flash

Pith's one-line read The paper proves that every irreducible representation attached to an elliptic Fargues–Scholze L-parameter yields a nonzero stable combination of Harish-Chandra characters, establishing stability of elliptic L-packets by geometric methods.

desk verdict Strong, original proof of stability for elliptic FS L-packets, endoscopy-free and uniform in G, but the standing assumption of an F-rational Borel containing T_g fails for anisotropic tori and is load-bearing. read the letter →

arxiv 2501.00652 v1 pith:LAJQFCGY submitted 2024-12-31 math.RT math.AGmath.NT

classification math.RTmath.AGmath.NT MSC 22E5011R3914D24
keywords Fargues-ScholzeL-packetsellipticL-parametersstableHarish-ChandracharacterslocalLanglandscorrespondencegeometrizationspectralactionweightmultiplicitiesp-adicreductivegroups
verification ladder T0 review T1 audit T2 compute T3 formal

The pith

A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.

The reading

Stability of $L$-packets is a central prediction of the local Langlands correspondence: the Harish-Chandra characters of members of a packet should admit a linear combination invariant under stable conjugacy. This paper proves that prediction for every elliptic $L$-parameter, using the Fargues–Scholze geometrization of representations as sheaves on the stack of $G$-bundles over the Fargues–Fontaine curve. For each irreducible $\pi$ with Fargues–Scholze $L$-parameter $\varphi$, the paper constructs a finite virtual representation $\pi_0$ by acting on $\pi$ with the regular representation of the centralizer $S_\varphi$ modulo the fixed center, and shows that the alternating sum of its Harish-Chandra characters is stable on elliptic regular semisimple elements. In characteristic zero the same object is a nonzero stable distribution on $G(F)$. The interest is a new route to stability that does not use endoscopic classification and works in positive characteristic.

What carries the argument

The load-bearing construction is the spectral action of Fargues–Scholze: perfect complexes on the stack of $L$-parameters act on the derived category of sheaves on $\mathrm{Bun}_G$, and the averaged object $(i_\varphi)_* O(S_\varphi/Z(\widehat G)^\Gamma) * (i_1)_! \pi$ is shown to be a Hecke eigensheaf, so $T_{V_\mu}$ multiplies it by $\dim V_\mu$. The Hansen–Kaletha–Weinstein formula rewrites the resulting character identity as a weighted sum over stable conjugacy classes, with weights $\dim V_\mu[\lambda]/\dim V_\mu$ indexed by an invariant in the finite abelian group $H_g = \ker(X_*(T_g)_\Gamma \to \pi_1(G)_\Gamma)$. Choosing $\mu = 4m\rho_G$ and applying the Weyl character formula, the paper proves that these weight multiplicities equidistribute over $H_g$ as $m \to \infty$; Fourier analysis on $H_g$ then makes the weighted sum independent of the class of $g'$, which is exactly stability.

What would settle it

For $G = \mathrm{GL}_2(\mathbb{Q}_p)$, take an elliptic regular element $g$ whose centralizer is a quadratic-field torus. Compute the difference $S_{h,m} - S_{h',m}$ for the two classes $h,h'$ in $H_g$: the proof requires this to tend to $0$, so a nonzero limit would falsify the equidistribution step. Separately, in a case where the Fargues–Scholze packet is known, compare $\Theta_{\pi_0}$ on two stably conjugate elliptic elements; unequal values would refute the stability conclusion.

Watch

Extended reading notes

Core claim

Let $G$ be a connected reductive group over a non-archimedean local field $F$, and let $\varphi: W_F \to \widehat G(\overline{\mathbb Q}_\ell)$ be an elliptic $L$-parameter. For every irreducible smooth representation $\pi$ of $G(F)$ whose Fargues–Scholze $L$-parameter is $\varphi$, the paper defines $F_0 = (i_\varphi)_* O(S_\varphi/Z(\widehat G)^\Gamma) * (i_1)_! \pi$ and $\pi_0 = i_1^* F_0$. It proves that $\pi_0$ is a finite direct sum of irreducible representations (up to degree shifts) containing $\pi$, and that the Harish-Chandra character $\Theta_{\pi_0}$ is a nonzero function on the elliptic regular semisimple locus $G(F)_{\mathrm{ell}}$ invariant under $G(\overline F)$-conjugacy. In characteristic zero $\Theta_{\pi_0}$ is a nonzero stable distribution on all of $G(F)$. This establishes the stability of the Fargues–Scholze $L$-packet $\Pi^{\mathrm{FS}}_\varphi(G)$ in the sense required by the stability conjecture, without invoking the theory of endoscopy.

Load-bearing premise

The proof assumes that for every elliptic regular element $g$ the centralizer torus $T_g$ has an $F$-rational Borel subgroup containing it; for non-split elliptic tori this can fail, and without a chosen Borel the weight multiplicities cannot be canonically compared across the stable conjugacy class.

Editorial extensions

If this is right

  • Every elliptic Fargues–Scholze packet, whenever nonempty, carries a nonzero stable character combination, so the stability part of the local Langlands stability conjecture holds for these packets.
  • The stable combination is canonically built from any member: $\Theta_{\pi_0}$ for $\pi_0 = O(S_\varphi/Z(\widehat G)^\Gamma) * \pi$, with coefficients coming from an equal-weight limit of weight multiplicities.
  • The method is independent of endoscopic classification and covers positive characteristic, where full endoscopy is not available.
  • The same weighted-sum identity transfers character values between extended pure inner forms, up to the sign $(-1)^{\langle \mu, 2\rho_G\rangle}$.
  • The equidistribution of weight multiplicities (Theorem 4.3.2) is a separate, self-contained result about highest-weight representations of reductive groups.

Reading between the lines

Editorial extensions of the paper, not claims the author makes directly.

  • If the Fargues–Scholze packet is nonempty for every elliptic parameter, the same construction would prove the stability conjecture for all elliptic discrete-series packets; the paper leaves nonemptiness open.
  • The regular representation $O(S_\varphi/Z(\widehat G)^\Gamma)$ may be the correct canonical packet average; in cases where a classical packet is known, this stable combination should agree with the classical stable packet character.
  • A testable extension is to compute $S_{h,m}$ explicitly for a small-rank split group and a non-split elliptic torus, to measure how quickly the equidistribution limit is approached.
Share X Bluesky LinkedIn Reddit HN

Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, and a circularity audit.

Referee Report

2 major / 3 minor

Summary. The paper proves stability of a virtual Harish-Chandra character attached to an elliptic Fargues-Scholze L-parameter. For an elliptic L-parameter ϕ and an irreducible smooth representation π of G(F) with Fargues-Scholze parameter ϕ, the author defines π0 := O(S_ϕ / Z(Ĝ)^Γ) * π via the spectral action, shows that the corresponding sheaf on Bun_G is a Hecke eigensheaf, and uses the Hansen-Kaletha-Weinstein character formula to express Θ_{π0}(g) as a weighted sum over the stable conjugacy class of an elliptic element g. The proof then reduces stability to an equidistribution statement for weight multiplicities of V_{μ_m}, proved by Fourier analysis on the finite abelian group H_g. The main theorem asserts that Θ_{π0} is stable on G(F)_ell and, in characteristic zero, is a non-zero stable distribution.

Significance. If correct, this provides a new, endoscopy-independent proof of stability for Fargues-Scholze L-packets, with the advertised advantage of working in positive characteristic. The argument is genuinely constructive and has no fitted parameters: the Hecke eigensheaf property is proved in Proposition 4.1.2, the equidistribution statement is proved in Theorem 4.3.1, and the use of the spectral action and HKW22 as external benchmarks avoids circularity. The main defect is a repeated false geometric assumption about the existence of an F-rational Borel subgroup containing an elliptic maximal torus; this affects the formulation of the weighted-sum formula and the equidistribution theorem, and therefore the proof of the main theorem as written.

major comments (2)
  1. [§1.1 Step 2; §4.2 before Corollary 4.2.8; §4.3 before Theorem 4.3.1] The proof repeatedly assumes that for every elliptic g ∈ G(F)_ell there exists a Borel subgroup defined over F containing T_g = Cent(g,G). This assumption is false in general. For example, in G=GL_2 over a non-archimedean local field, an element whose centralizer is the unramified quadratic torus is elliptic, but an F-rational Borel subgroup contains only split maximal tori. Consequently, the identification X_*(T_g) ≅ X_*(T_univ) with a distinguished dominance order, the definition of H_g, and the reindexing λ=inv(g,g') in Corollary 4.2.8 are not justified for such elements. Since the weighted-sum formula (19) and the quantities S_{h,m} in Theorem 4.3.1 are the inputs to the stability proof, Theorem 4.3.3 is not proven for elliptic elements with anisotropic centralizer as written. The text supplies no alternative construction for groups or elements where the assumption fails.
  2. [§3.2, Lemma 3.2.2] The surjection Λ^Φ → H_g used in Proposition 3.3.1(2) is constructed via a non-canonical isomorphism X_*((T_g)_sc) ≅ Λ^Φ that is explicitly noted not to be Γ-equivariant. The paper does not prove that the resulting character χ of Λ^Φ, and hence the existence of β with χ(β) ≠ 1, is independent of this choice. While the conclusion is likely true because any two choices differ by a Weyl-group element and the weight multiplicities of V_{μ_m} are W-invariant, this independence is not stated or proved; as written, the growth estimate depends on choices that are not shown to be canonical.
minor comments (3)
  1. [Throughout] Several typographical errors should be fixed: "a prior" should be "a priori" (e.g., in §1.2 and Remark 4.3.4), "Combing Corollary 4.2.8" should be "Combining", and "charater" in Lemma 4.3.5 should be "character".
  2. [Introduction, after Equation (9)] The line "π0 = i_1^*F0 F0 ≅ i_1!π0" appears garbled; it should read "π0 := i_1^*F0, and F0 ≅ i_1!π0".
  3. [§1.1, Step 2] The phrase "We choose a Borel subgroup over F containing (T_g)_F" is not just notationally strong but mathematically impossible for anisotropic elliptic tori; as noted in the major comments, this needs to be replaced by an admissible embedding or by a Borel over an algebraic closure with an explicit independence statement.

Circularity Check

0 steps flagged · score 0.0 of 10

No significant circularity: the stability theorem is derived from the external spectral action, the HKW22 transfer formula, and the Weyl character formula, none of which assume the target result.

full rationale

The paper's derivation chain is self-contained against external benchmarks rather than circular. The object π0 is defined by the spectral action of the regular representation O(Sϕ) on π, and the Hecke eigensheaf property (Proposition 4.1.2) is proved from the projection formula and the semisimplicity of the regular representation, not from stability. The weighted-sum formula (Corollary 4.2.8) is obtained by combining this eigensheaf property with the Hansen–Kaletha–Weinstein transfer formula [HKW22, Theorem 6.5.2], which is an independent external result about local shtuka spaces. The equidistribution step (Theorem 4.3.1 and Theorem 4.3.2) is a genuine asymptotic statement about weight multiplicities of highest weight representations, proved via the Weyl character formula and Fourier analysis on the finite abelian group Hg; no fitted parameter or hidden input is renamed as a prediction. The theorem's conclusion — stability of Θπ0 — is not assumed at any point; it is derived after the limit of the coefficients is shown to be independent of the representative g′. The paper explicitly notes in Section 1.2 that it proves something different from classical endoscopy-based results, and no load-bearing self-citation chain appears: the main cited inputs [FS21] and [HKW22] are external and do not include the stability theorem. The skeptical concern about the assumption of an F-rational Borel containing T_g is a mathematical correctness issue about a genuinely needed hypothesis, not a circularity issue, because the assumption does not encode the conclusion. Therefore no circular step is present, and the score is 0.

Assumptions & free parameters 0 free parameters · 5 assumptions · 0 invented entities

The central claim relies on a large external machinery: Fargues-Scholze's geometrization (spectral action, Hecke operators, L-parameter stack), Hansen-Kaletha-Weinstein's trace formula for local shtuka spaces, Harish-Chandra character theory, and Arthur's stabilization. These are treated as black boxes. No free parameters are introduced and no new entities are postulated.

assumptions (5)
  • domain assumption Fargues-Scholze spectral action exists and is compatible with Hecke operators (FS21, Corollary X.1.3).
    Used to define pi0 and prove the Hecke eigensheaf property (Prop 4.1.2, Lemma 4.2.2).
  • domain assumption Hansen-Kaletha-Weinstein trace formula (Theorem 2.6.1) computes the Harish-Chandra character of the Hecke correspondence.
    Establishes Equation (18) and the weighted-sum expression (Cor 4.2.8).
  • domain assumption Harish-Chandra character theory (local integrability, linear independence) holds over Q_l and in positive characteristic.
    Needed to define Theta_pi0 and to deduce non-vanishing from linear independence (Lemma 4.3.5).
  • domain assumption Arthur's stabilization theorem (Art96, Theorem 6.1) for elliptic virtual characters.
    Upgrades stability on elliptic elements to a stable distribution in characteristic zero (Theorem 4.3.6).
  • standard math Standard structure of reductive groups over local fields: Borovoi fundamental group, Kottwitz set, elliptic tori.
    Background for H_g, B(T_g), and the Fourier argument.

how reviews work

0 comments
Cite this review

Pith. "Pith review of Stability of Elliptic Fargues-Scholze $L$-packets." pith.science (2026). https://pith.science/paper/LAJQFCGY

@misc{pith2026250100652,
  author       = {Pith},
  title        = {Pith review of: Stability of Elliptic Fargues-Scholze $L$-packets},
  year         = {2026},
  howpublished = {\url{https://pith.science/paper/LAJQFCGY}},
  note         = {Machine review of arXiv:2501.00652}
}
abstract

Let $F$ be a non-archimedean local field. Let $\overline{F}$ be an algebraic closure of $F$. Let $G$ be a connected reductive group over $F$. Let $\varphi$ be an elliptic $L$-parameter. For every irreducible representation $\pi$ of $G(F)$ with Fargues--Scholze $L$-parameter $\varphi$, we prove that there exists a finite set of irreducible representations $\{\pi_i\}_{i \in I}$ containing $\pi$, such that $\pi_i$ has Fargues--Scholze $L$-parameter $\varphi$ for all $i \in I$ and a certain non-zero $\mathbb{Z}$-linear combination $\Theta_{\pi_0}$ of the Harish-Chandra characters of $\{\pi_i\}_{i \in I}$ is stable under $G(\overline{F})$ conjugation, as a function on the elliptic regular semisimple elements of $G(F)$. Moreover, if $F$ has characteristic zero, $\Theta_{\pi_0}$ is a non-zero stable distribution on $G(F)$.

Discussion (0). Continue with ORCID to comment.

Reference graph

Works this paper leans on

34 extracted references · 28 canonical work pages

  1. [1]

    On local character relations

    James Arthur. On local character relations. Sel. Math., New Ser. , 2(4):501--579, 1996

  2. [2]

    The endoscopic classification of representations

    James Arthur. The endoscopic classification of representations. Orthogonal and symplectic groups , volume 61 of Colloq. Publ., Am. Math. Soc. Providence, RI: American Mathematical Society (AMS), 2013

  3. [3]

    Adler and Loren Spice

    Jeffrey D. Adler and Loren Spice. Supercuspidal characters of reductive \(p\) -adic groups. Am. J. Math. , 131(4):1137--1210, 2009

  4. [4]

    Gonz \'a lez-Avil \'e s

    Mikhail Borovoi and Cristian D. Gonz \'a lez-Avil \'e s. The algebraic fundamental group of a reductive group scheme over an arbitrary base scheme. Cent. Eur. J. Math. , 12(4):545--558, 2014

  5. [5]

    Galois action on B orovoi's algebraic fundamental group

    Mikhail Borovoi (https://mathoverflow.net/users/4149/mikhail borovoi). Galois action on B orovoi's algebraic fundamental group. MathOverflow. url: https://mathoverflow.net/q/484122 (version: 2024-12-15)

  6. [6]

    Abelian Galois cohomology of reductive groups , volume 626 of Mem

    Mikhail Borovoi. Abelian Galois cohomology of reductive groups , volume 626 of Mem. Am. Math. Soc. Providence, RI: American Mathematical Society (AMS), 1998

  7. [7]

    Affine Springer fibers and depth zero L-packets

    Roman Bezrukavnikov and Yakov Varshavsky. Affine Springer fibers and depth zero L -packets. Preprint, arXiv :2104.13123 [math. RT ] (2021), 2021

  8. [8]

    Introduction to admissible representations of p-adic groups

    Bill Casselman. Introduction to admissible representations of p-adic groups. unpublished notes , 1995. url: https://www.math.utah.edu/ ptrapa/math-library/casselman/casselman-p-adic-book.pdf

Show all 34 references
  1. [9]

    Moduli of L anglands parameters

    Jean-Fran c ois Dat, David Helm, Robert Kurinczuk, and Gilbert Moss. Moduli of L anglands parameters. arXiv preprint arXiv:2009.06708 , 2020

  2. [10]

    Depth-zero supercuspidal \(L\) -packets and their stability

    Stephen DeBacker and Mark Reeder. Depth-zero supercuspidal \(L\) -packets and their stability. Ann. Math. (2) , 169(3):795--901, 2009

  3. [11]

    Stability of character sums for positive-depth, supercuspidal representations

    Stephen DeBacker and Loren Spice. Stability of character sums for positive-depth, supercuspidal representations. J. Reine Angew. Math. , 742:47--78, 2018

  4. [12]

    Representation theory

    William Fulton and Joe Harris. Representation theory. A first course , volume 129 of Grad. Texts Math. New York etc.: Springer-Verlag, 1991

  5. [13]

    Supercuspidal representations: construction, classification, and characters

    Jessica Fintzen. Supercuspidal representations: construction, classification, and characters. url: https://www.math.uni-bonn.de/people/fintzen/IHES_Fintzen.pdf

  6. [14]

    A twisted Yu construction, Harish - Chandra characters, and endoscopy

    Jessica Fintzen, Tasho Kaletha, and Loren Spice. A twisted Yu construction, Harish - Chandra characters, and endoscopy. Duke Math. J. , 172(12):2241--2301, 2023

  7. [15]

    Geometrization of the local L anglands correspondence

    Laurent Fargues and Peter Scholze. Geometrization of the local L anglands correspondence. arXiv preprint arXiv:2102.13459 , 2021

  8. [16]

    The local Langlands conjecture for \( GSp (4)\)

    Wee Teck Gan and Shuichiro Takeda. The local Langlands conjecture for \( GSp (4)\) . Ann. Math. (2) , 173(3):1841--1882, 2011

  9. [17]

    Beijing notes on the categorical local langlands conjecture

    David Hansen. Beijing notes on the categorical local langlands conjecture. arXiv preprint arXiv:2310.04533 , 2023

  10. [18]

    Admissible invariant distributions on reductive \(p\) -adic groups

    Harish-Chandra. Admissible invariant distributions on reductive \(p\) -adic groups. Notes by Stephen DeBacker and Paul J . Sally jun , volume 16 of Univ. Lect. Ser. Providence, RI: American Mathematical Society, 1999

  11. [19]

    On the Kottwitz conjecture for local shtuka spaces

    David Hansen, Tasho Kaletha, and Jared Weinstein. On the Kottwitz conjecture for local shtuka spaces. Forum Math. Pi , 10:79, 2022. Id/No e13

  12. [20]

    Humphreys

    James E. Humphreys. Introduction to Lie algebras and representation theory. 3rd printing, rev , volume 9 of Grad. Texts Math. Springer, Cham, 1980

  13. [21]

    Representations of algebraic groups

    Jens Carsten Jantzen. Representations of algebraic groups. , volume 107 of Math. Surv. Monogr. Providence, RI: American Mathematical Society (AMS), 2nd ed. edition, 2003

  14. [22]

    Regular supercuspidal representations

    Tasho Kaletha. Regular supercuspidal representations. J. Am. Math. Soc. , 32(4):1071--1170, 2019

  15. [23]

    Representations of reductive groups over local fields

    Tasho Kaletha. Representations of reductive groups over local fields. arXiv preprint arXiv:2201.07741 , 2022

  16. [24]

    Kottwitz

    Robert E. Kottwitz. Isocrystals with additional structure. Compos. Math. , 56:201--220, 1985

  17. [25]

    B( G ) for all local and global fields

    Robert Kottwitz. B( G ) for all local and global fields. Preprint, arXiv :1401.5728 [math. RT ] (2014), 2014

  18. [26]

    The local langlands conjecture

    Tasho Kaletha and Olivier Ta bi. The local langlands conjecture. In https://otaibi.perso.math.cnrs.fr/kaletha-taibi-llc.pdf

  19. [27]

    Endoscopic decomposition of certain depth zero representations

    David Kazhdan and Yakov Varshavsky. Endoscopic decomposition of certain depth zero representations. In Studies in Lie theory. Dedicated to A. Joseph on his sixtieth birthday , pages 223--301. Basel: Birkh \"a user, 2006

  20. [28]

    Stabilisation de la formule des traces tordue

    Colette Moeglin and Jean-Loup Waldspurger. Stabilisation de la formule des traces tordue. Vol . 2 , volume 317 of Prog. Math. Basel: Birkh \"a user/Springer, 2016

  21. [29]

    Explicit asymptotic expansions for tame supercuspidal characters

    Loren Spice. Explicit asymptotic expansions for tame supercuspidal characters. Compos. Math. , 154(11):2305--2378, 2018

  22. [30]

    Explicit asymptotic expansions in p-adic harmonic analysis II

    Loren Spice. Explicit asymptotic expansions in p-adic harmonic analysis II . Preprint, arXiv :2108.12935 [math. RT ] (2021), 2021

  23. [31]

    T. A. Springer. Reductive groups. Automorphic forms, representations and L -functions, Proc . Symp . Pure Math . Am . Math . Soc ., Corvallis / Oregon 1977, Proc . Symp . Pure Math . 33, 1, 3-27 (1979)., 1979

  24. [32]

    The Jacquet--Langlands correspondence for GL_2( Q _p)

    Olivier Ta bi. The Jacquet--Langlands correspondence for GL_2( Q _p) . Notes for the M2 course

  25. [33]

    Some comments on the stable bernstein center

    Sandeep Varma. Some comments on the stable bernstein center

  26. [34]

    The categorical form of F argues' conjecture for tori

    Konrad Zou. The categorical form of F argues' conjecture for tori. arXiv preprint arXiv:2202.13238 , 2022

Pith tools

Reviewed August 10, 2026 · model on record in the stance chip above.