REVIEW 3 major objections 4 minor 35 references
Homological vanishing for the Steinberg representation II: reductive groups and integral conjectures
T0 review · 3 major / 4 minor · reviewed 2026-08-05 · deepseek-v4-flash
Pith's one-line read Steinberg homology vanishes in a range for every reductive group.
desk verdict Serious, substantial paper with Theorem B extending Steinberg vanishing to all reductive groups; Theorem D is new but rests on a deferred exactness proof and a thinly sketched case, so it needs revision, not desk rejection. read the letter →
The pith
A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.
The reading
What carries the argument
The engine is a resolution of St(G) whose degree-i term is a sum, over standard Levi subgroups obtained by deleting i+1 simple roots, of induced Steinberg representations of those Levi subgroups. Feeding this resolution into group homology gives a spectral sequence whose E^1 page is built from smaller-rank groups; the induction is driven by the parabolic induction formula for Steinberg representations and by vanishing and surjectivity statements for the end Levi factors. For the integer theorems the load-bearing object is the double Tits building T^2(Z^n), the simplicial complex of pairs of compatible flags of direct summands; a prior theorem identifies its homology with that of the bar reso
What would settle it
Search the finite generating sets of Examples 31.3 and 31.4 for rank 4 or 5: if any element of X_1(V) is killed by the boundary to X_0(V) but is not in the image of the boundary from X_2(V), then Theorem 31.5 and hence Theorem D are false. Equivalently, a direct computation of H_1 of the bar resolution S_bullet(Z^4) that produced a nonzero class would settle the question in the negative.
Extended reading notes
Core claim
On the paper's own terms, the discovery is that homological vanishing for Steinberg representations is a formal consequence of the structure of relative root systems, not of special features of split groups. Theorem B states that H_i(G(k); St(G;F)) = 0 for i at most b(Phi_k(G)), with b(A_n) = floor((n-1)/2), b(B_n) = b(C_n) = b(BC_n) = floor((n-2)/2), b(D_n) = floor((n-3)/2), b = 0 for exceptional systems, and for reducible systems a sum of the component bounds plus m-1. In the integral setting, Theorem D states that the double Tits building T^2(Z^n) is n-connected for n at least 4, and combined with a conditional spectral sequence it yields Corollary E: H_1 and H_2 with Steinberg coefficien
Load-bearing premise
For the integral half, the load-bearing premise is the exactness of the truncated three-step resolution X_2(V) -> X_1(V) -> X_0(V) -> St(V) -> 0; the paper verifies it by saying the generators and relations match a cited source and that enlarging X_2 does not affect the claim, rather than giving a full detailed proof, and if this exactness fails the connectivity of T^2(Z^n) would collapse.
Editorial extensions
If this is right
- Theorem B gives a vanishing range for nonsplit reductive groups, so Steinberg-cohomology vanishing now applies to every connected reductive group over any field.
- The proof replaces topology of partial flag complexes with a self-contained spectral sequence from a Levi-subgroup resolution, and the same machinery drives the integral results.
- For exceptional relative root systems the theorem yields only H_0 vanishing, but the paper notes that a less uniform argument would raise the bounds for E_6, E_7, and E_8.
- The n-connectivity of T^2(Z^n) for n at least 4 yields new integral vanishing: H_1 for SL_n(Z) and GL_n(Z) in ranges starting near n=4 or 5, and H_2 near n=6 or 8, depending on whether 2 and 3 are invertible in the coefficient ring.
- If the connectivity assumption in Theorem C holds for larger b, the same mechanism proves the conjectured stable-range vanishing for SL_n(Z) and GL_n(Z) up to degree b for large n.
Reading between the lines
- My inference: the relative-root-system formulation suggests analogous integral vanishings for other reductive Z-forms, such as symplectic or orthogonal groups, should follow from a connectivity conjecture for the appropriate integral double buildings; the paper explicitly says the approach can be generalized but does not write the details.
- My inference: the 1/3 slope in the integral theorem appears forced by the base case SL_3(Z), where surjectivity is only verified after inverting 2 and 3; a sharper base case or a different resolution might make a 1/2 slope accessible, but the paper's own remark indicates its spectral sequence alone cannot achieve that.
- My inference: the combinatorial restatement in Lemma 33.1 turns each summand of the connectivity proof into a finite statement about partitions of a set respecting specified subsets, so low-rank cases could be checked by computer search and might indicate whether higher cases of the b-integral resolution conjecture hold.
- My inference: a natural testable extension is to replace Z by Z[1/N] or rings of integers in number fields; the bar-resolution formalism and the double-building definition carry over, and the first obstruction would be the analogue of the three-step partial resolution for those coefficient rings.
Signed reviews
Editorial analysis
A structured set of objections, weighed in public.
Referee Report
Summary. The paper proves that for any connected reductive group G over a field k, the homology of G(k) with coefficients in the Steinberg representation vanishes in a range depending only on the relative root system Φ_k(G) (Theorem B). For the classical split groups this was known by Ash–Putman–Sam; the new proof is uniform and extends to all reductive groups, including nonsplit forms and the nonreduced system BC_n. The paper also states an integral refinement: assuming a high-connectivity conjecture for the double Tits building T^2(Z^n), it obtains vanishing for H_i(SL_n(Z); St) and H_i(GL_n(Z); St) in a range that improves with the connectivity assumption (Theorem C). The main new unconditional evidence is Theorem D, asserting that T^2(Z^n) is n-connected for n ≥ 4, which yields, via Theorem C, new low-degree vanishing for SL_n(Z) and GL_n(Z) (Corollary E). The proofs use a resolution of the Steinberg representation, a spectral sequence built from Levi subgroups, and, for the integral part, a double complex of partial resolutions.
Significance. If the gaps identified below are repaired, this is a substantial contribution. Theorem B unifies and extends prior vanishing results to all reductive groups over fields, with explicit bounds depending on the relative root system; the spectral sequence machinery and the explicit differential calculations in Parts 2–4 are detailed and appear sound. The integral part gives a clean conditional framework for the Church–Farb–Putman conjecture and proves a genuine new connectivity result for the double Tits building, leading to new low-degree vanishing for SL_n(Z) and GL_n(Z). The paper is careful to attribute prior work and states the precise dependency on external results (Reeder’s theorem, Solomon–Tits, and Miller–Patzt–Wilson’s identification of the bar resolution with the double Tits building). These strengths are real, but the load-bearing omissions discussed in the major comments must be addressed before the result can be considered fully verified.
major comments (3)
- [§31.3, Theorem 31.5; used in §32.2, Lemma 32.2] The exactness of X2(V)→X1(V)→X0(V)→St(V)→0 is asserted by comparison with [9], with the comment that enlarging X2 does not affect the claim because no assertion is made about ker(X2→X1). This is load-bearing: Lemma 32.2 requires H_i(X•(V))=0 for i=0,1 for every summand, and Theorem D′ (hence Theorem D and Corollary E) would fail if H_1(X•(V)) were nonzero. The argument is valid only if the enlarged X2(V) is a genuine chain complex with δ²=0 and its image lies in ker(X1→X0), and if the image of the [9] X2-term already covers that kernel. None of these is demonstrated; the text refers only to Examples 31.2–31.4. Please provide a direct verification for the new generators (especially the multi-line forms in Example 31.4) or quote and verify the precise statement from [9].
- [§25.4 (Theorem C′.1)] The proof of Theorem C′.1 is omitted with the note that it follows Part 2 closely after replacing the spectral sequence and the reducible-Levi results. This theorem supplies the 2-and-3-invertible cases of Corollary E, one of the paper's main concrete applications. The listed replacements are plausible, but the key differential lemmas (the analogues of Lemmas 12.1–12.3) are not stated or checked, and the induction with the cap min(b,·) needs verification. I recommend writing out the full proof or at least an appendix containing the integral analogues of the differential lemmas with precise statements.
- [§1.9, Theorem C] The statement of Theorem C gives the general-coefficient vanishing range as i ≤ min(b, ⌊(n−3)/3⌋). This is inconsistent with Theorem C′ (§25.2), which for GL_{n+1} gives i ≤ min(b, ⌊(n−1)/3⌋), i.e., for GL_n, i ≤ min(b, ⌊(n−2)/3⌋). Corollary E's thresholds n≥5 for i=1 and n≥8 for i=2 match the latter. The formula in Theorem C should be corrected to ⌊(n−2)/3⌋ (or the statement of Theorem C′ should be aligned if the intended bound is different).
minor comments (4)
- [Abstract] Typo: “The generalizes work” should be “This generalizes work”; “first and second of homology” should be “first and second homology groups.”
- [§1.9, Remark 1.18] The table and surrounding text are accurate but the sentence “In fact, with only a little more effort the proofs in [9, 15] work for F a commutative ring in which all primes p ≤ n are invertible” is slightly imprecise: the statement is correct for the specific claims used, but the reader must infer the exact scope. Consider making the coefficient condition explicit in the table.
- [§30.4, formula for ∂] The boundary formula for the bar complex is written with signs (−1)^{j−1}; this is fine, but the signs in the double-complex differential δ in §32.2 (with the shifts by r_1+...+r_{j−1}+i_1+...+i_{j−1}) deserve a small justification, especially because Lemma 32.2 relies on the signed differentials being compatible with the tensor-product differentials. Currently the verification is only hinted at.
- [Throughout] The paper is very long and some parts are repetitive (e.g., the three parallel treatments of types A, B/C/BC, and D). This is a presentation issue rather than a correctness issue.
Circularity Check
No significant circularity: the main derivation is self-contained and the cited same-author results are used as independent external theorems; the terse proof of Theorem 31.5 is a verification risk, not a circular reduction.
full rationale
The paper's central vanishing theorem (Theorem B) is proved from a resolution of the Steinberg representation, the Solomon–Tits theorem, Reeder's induction theorem, standard reductive-group structure theory, and an induction on rank with explicit differential computations. None of these inputs is defined in terms of the vanishing conclusion, and no fitted parameter is later renamed as a prediction. The integral part is explicitly conditional: Theorem C assumes a connectivity hypothesis, and Theorem D proves a special case of that hypothesis. Theorem D does not assume the n-connectivity of T^2(Zn); instead it proves exactness of a three-step resolution and compares double complexes. The load-bearing inputs Theorem 23.3 (from Miller–Patzt–Wilson [22]) and Theorem 31.5 (from Brück–Miller–Patzt–Sroka–Wilson [9]) are external published results with overlapping authorship, but their statements do not include the target connectivity theorem and they are used as black boxes with independent proofs. The proof of Theorem 31.5 is notably terse: it says the enlarged X2 does not affect the exactness proved in [9], without displaying the verification that the added X2 generators lie in the kernel of the X1 differential. If that exactness failed, Lemma 32.2 and hence Theorem D would fail; this is a genuine correctness/verification risk in the manuscript, but it is not a circularity because the argument is an appeal to an external theorem, not an assumption of the theorem being proved. No uniqueness theorem is imported to forbid alternatives, and no known result is merely renamed. Accordingly, the score stays in the non-circular 0–2 range, with the small elevation reflecting the same-author citations and the terse delegation rather than any reduction-by-construction.
Assumptions & free parameters
assumptions (5)
- standard math Solomon-Tits theorem: the Tits building T(G) is homotopy equivalent to a wedge of (n-1)-spheres, so St(G;F) = H_{n-1}(T(G);F).
- standard math Borel-Tits structure theory for reductive groups: existence and conjugacy of parabolic subgroups, Levi decomposition, relative root systems, BN-pairs, Bruhat decomposition, and root subgroups.
- domain assumption Reeder map (Theorem 2.7): for a parabolic P with Levi factor L, St(G)|_{P(k)} is isomorphic as a P(k)-module to Ind_{L(k)}^{P(k)} St(L).
- domain assumption Miller-Patzt-Wilson Theorem 23.3: H_i(S_*(Z^{n+1})) is isomorphic to H_{i+n}(T^2(Z^{n+1})) for the bar resolution of the Steinberg module.
- domain assumption Bruck-Miller-Patzt-Sroka-Wilson resolution: the two-step presentation X_1(V) -> X_0(V) -> St(V) -> 0 is exact, and the variant Theorem 31.5 extends it to a three-step complex with X_2(V).
Cite this review
Pith. "Pith review of Homological vanishing for the Steinberg representation II: reductive groups and integral conjectures." pith.science (2026). https://pith.science/paper/UWW5BKZZ
@misc{pith2026250901559,
author = {Pith},
title = {Pith review of: Homological vanishing for the Steinberg representation II: reductive groups and integral conjectures},
year = {2026},
howpublished = {\url{https://pith.science/paper/UWW5BKZZ}},
note = {Machine review of arXiv:2509.01559}
}
abstract
We prove that the homology groups of any connected reductive group over a field with coefficients in the Steinberg representation vanish in a range. The generalizes work of Ash-Putman-Sam on the classical split groups. We state a connectivity conjecture that would allow us to prove such a vanishing result for $SL_n(\mathbb{Z})$, as was conjectured by Church-Farb-Putman. We prove some special cases of this conjecture and use it to refine known results about the first and second of homology of $SL_n(\mathbb{Z})$ with Steinberg coefficients.
Reference graph
Works this paper leans on
-
[9]
B. Br¨ uck, J. Miller, P. Patzt, R. Sroka, & J. Wilson, On the codimension-two cohomology of SL n(Z), Adv. Math. 451 (2024), 109795. (Cited on pages 5, 7, 64, and 66.)
work page 2024
- [22]
-
[1]
A. Ash, J. Miller, & P. Patzt, Hopf algebras, Steinberg modules, and the unstable cohomology of SLn(Z) and GL n(Z), preprint 2024. arXiv:2404.13776 (Cited on page 63.)
arXiv 2024
-
[2]
A. Ash, A. Putman, & S. Sam, Homological vanishing for the Steinberg representation, Compos. Math. 154 (2018), no. 6, 1111–1130. (Cited on pages 2, 5, and 6.)
work page 2018
-
[3]
A. Ash & L. Rudolph, The modular symbol and continued fractions in higher dimensions, Invent. Math. 55 (1979), no. 3, 241–250. (Cited on page 63.)
work page 1979
-
[4]
A. Borel, Linear algebraic groups, second edition, Graduate Texts in Mathematics, 126, Springer-Verlag, New York, 1991. (Cited on pages 2, 7, 22, and 23.) HOMOLOGICAL V ANISHING FOR THE STEINBERG REPRESENTATION II 73
work page 1991
-
[5]
A. Borel & J.-P. Serre, Corners and arithmetic groups, Comment. Math. Helv. 48 (1973), 436–491. (Cited on page 5.)
work page 1973
- [6]
Show all 35 references
-
[7]
Groupes r´ eductifs
A. Borel & J. Tits, Compl´ ements ` a l’article: “Groupes r´ eductifs”, Inst. Hautes´Etudes Sci. Publ. Math. No. 41 (1972), 253–276. (Cited on page 7.)
1972
-
[8]
K. S. Brown, Buildings, Springer, New York, 1989. (Cited on pages 3, 8, 9, and 16.)
1989
-
[10]
Br¨ uck, P
B. Br¨ uck, P. Patzt, & R. Sroka, A presentation of symplectic Steinberg modules and cohomology of Sp2g(Z), preprint 2023. arXiv:2306.03180 (Cited on page 5.)
2023 arXiv
-
[11]
Br¨ uck, Y
B. Br¨ uck, Y. Santos Rego, & R. Sroka, On the top-dimensional cohomology of arithmetic Chevalley groups, Proc. Amer. Math. Soc. 152 (2024), 4131–4139. (Cited on page 5.)
2024
-
[12]
Bykovskii, Generating elements of the annihilating ideal for modular symbols, Funktsional
V.A. Bykovskii, Generating elements of the annihilating ideal for modular symbols, Funktsional. Anal. i Prilozhen. 37 (2003), no. 4, 27–38, 95; translation in Funct. Anal. Appl. 37 (2003), no. 4, 263–272. (Cited on pages 64 and 66.)
2003
-
[13]
Charlton, D
S. Charlton, D. Radchenko, & D. Rudenko, Multiple polylogarithms and the Steinberg module, preprint
-
[14]
Algebraic Topology: Applications and New Directions
T. Church, B. Farb, & A. Putman, A stability conjecture for the unstable cohomology of SL n(Z), mapping class groups, and Aut(Fn), in “Algebraic Topology: Applications and New Directions”, 55–70, Contemp. Math. 620 (2014), Amer. Math. Soc., Providence, RI. (Cited on pages 2, 5...
2014
-
[15]
Church & A
T. Church & A. Putman, The codimension-one cohomology of SL n Z, Geom. Topol. 21 (2017), no. 2, 999–1032. (Cited on pages 5, 7, 56, 64, and 66.)
2017
-
[16]
C. W. Curtis, The Steinberg character of a finite group with a ( B, N)-pair, J. Algebra 4 (1966), 433–441. (Cited on page 3.)
1966
-
[17]
J. E. Humphreys, The Steinberg representation, Bull. Amer. Math. Soc. (N.S.) 16 (1987), no. 2, 247–263. (Cited on page 3.)
1987
-
[18]
Lee & R.H
R. Lee & R.H. Szczarba, On the homology and cohomology of congruence subgroups, Invent. Math. 33 (1976) 1, 15–53. (Cited on pages 5, 7, and 56.)
1976
-
[19]
Miller, R
J. Miller, R. Nagpal, & P. Patzt, Stability in the high-dimensional cohomology of congruence subgroups, Compos. Math. 156 (2020), no. 4, 822–861. (Cited on pages 6, 7, 18, 49, and 56.)
2020
-
[20]
Stability in the high-dimensional cohomology of congruence subgroups
J. Miller, R. Nagpal, & P. Patzt, Corrigendum to “Stability in the high-dimensional cohomology of congruence subgroups” [Compos. Math. 156 (2020), no. 4, 822–861] arXiv:2508.14945 (Cited on page 18.)
2020 arXiv
-
[21]
Miller, P
J. Miller, P. Patzt, & A. Putman, The double Tits building and the bar resolution of the Steinberg representation, in preparation. (Cited on page 49.)
-
[23]
J. S. Milne, Algebraic groups, Cambridge Studies in Advanced Mathematics, 170, Cambridge Univ. Press, Cambridge, 2017. (Cited on pages 2, 7, and 22.)
2017
-
[24]
Putman & A
A. Putman & A. Snowden, The Steinberg representation is irreducible, Duke Math. J. 172 (2023), no. 4, 775–808. (Cited on page 3.)
2023
-
[25]
Reeder, The Steinberg module and the cohomology of arithmetic groups, J
M. Reeder, The Steinberg module and the cohomology of arithmetic groups, J. Algebra 141 (1991), no. 2, 287–315. (Cited on page 10.)
1991
-
[26]
Rognes, A spectrum level rank filtration in algebraic K-theory, Topology 31 (1992), no
J. Rognes, A spectrum level rank filtration in algebraic K-theory, Topology 31 (1992), no. 4, 813–845. (Cited on pages 5, 6, and 49.)
1992
-
[27]
Sikiri´ c, P
M. Sikiri´ c, P. Elbaz-Vincent, A. Kupers, & J. Martinet, Voronoi complexes in higher dimensions, cohomology of GLN (Z) for N ≥ 8, and the triviality of K8(Z), preprint 2019. arXiv:1910.11598 (Cited on page 56.)
2019 arXiv
-
[28]
Solomon, The Steinberg character of a finite group with BN -pair, in Theory of Finite Groups (Symposium, Harvard Univ., Cambridge, Mass., 1968), 213–221, Benjamin, New York
L. Solomon, The Steinberg character of a finite group with BN -pair, in Theory of Finite Groups (Symposium, Harvard Univ., Cambridge, Mass., 1968), 213–221, Benjamin, New York. (Cited on page 3.)
1968
-
[29]
Steinberg, A geometric approach to the representations of the full linear group over a Galois field, Trans
R. Steinberg, A geometric approach to the representations of the full linear group over a Galois field, Trans. Amer. Math. Soc. 71 (1951), 274–282. (Cited on page 3.)
1951
-
[30]
Steinberg, Prime power representations of finite linear groups, Canadian J
R. Steinberg, Prime power representations of finite linear groups, Canadian J. Math. 8 (1956), 580–591. (Cited on page 3.)
1956
-
[31]
Steinberg, Prime power representations of finite linear groups
R. Steinberg, Prime power representations of finite linear groups. II, Canadian J. Math. 9 (1957), 347–351. (Cited on page 3.)
1957
-
[32]
Steinberg, Comments on the Papers, in Robert Steinberg, Collected Works, 7, American Mathematical Society, Providence, RI, 1997
R. Steinberg, Comments on the Papers, in Robert Steinberg, Collected Works, 7, American Mathematical Society, Providence, RI, 1997. (Cited on page 3.) 74 JEREMY MILLER, PETER PATZT, AND ANDREW PUTMAN
1997
-
[33]
Tits, Classification of algebraic semisimple groups, in Algebraic Groups and Discontinuous Subgroups (Proc
J. Tits, Classification of algebraic semisimple groups, in Algebraic Groups and Discontinuous Subgroups (Proc. Sympos. Pure Math., Boulder, Colo., 1965), pp. 33–62 American Mathematical Society, Providence, RI, 1966. (Cited on page 11.)
1965
-
[34]
Tits, Buildings of spherical type and finite BN-pairs, Lecture Notes in Mathematics, Vol
J. Tits, Buildings of spherical type and finite BN-pairs, Lecture Notes in Mathematics, Vol. 386, Springer-Verlag, Berlin, 1974. (Cited on pages 3 and 8.) Dept of Mathematics; Purdue University; 150 N. University; West Lafayette, IN 47907 Email address: jeremykmiller@purdue.ed...
1974
-
[2025]
arXiv:2505.02202 (Cited on pages 18 and 66.)
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