REVIEW 4 major objections 5 minor 3 cited by
Auslander regular algebras and Coxeter matrices
T0 review · 4 major / 5 minor · reviewed 2026-08-10 · deepseek-v4-flash
Pith's one-line read The paper shows that the grade bijection of an Auslander regular algebra coincides with the permutation matrix in the Bruhat decomposition of its Coxeter matrix, and derives consequences for permanents, distributive lattices, and category…
desk verdict A genuinely new and mostly clean paper identifying the grade bijection with the Bruhat permutation of the Coxeter matrix, with one load-bearing citation to a proof-part of Iyama that should be promoted to a lemma. 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 load-bearing identity is the Bruhat factorization $C = P U_2$. The proof identifies the Coxeter matrix $C = -\omega^T \omega^{-1}$ with the matrix $M$ recording, with signs, which indecomposable injectives appear in the minimal injective coresolutions of indecomposable projectives; the row/column dimension-vector relation then gives $M = -C$. The admissible ordering of simples (grades nonincreasing along the order) makes the matrix $\varphi^{-1}M$ upper triangular, using the key property that the grade bijection $\varphi$ sends grade to cograde. The permutation matrix $P$ in the Bruhat decomposition is thus forced to be $\varphi$.
What would settle it
Compute the permanent of the Coxeter matrix of any finite-dimensional algebra of finite global dimension; if an Auslander regular algebra has a Coxeter matrix with permanent other than 1 or −1, the central theorem fails, and since the permanent is independent of the ordering of simples, a single such example suffices.
Extended reading notes
Core claim
The central claim is that for a finite-dimensional Auslander regular algebra, the grade bijection—the map sending a simple module to the top of the socle of its first nonzero Ext against the algebra—is encoded verbatim by linear algebra. When the simple modules are ordered so that grades increase with index, the Coxeter matrix factorizes as $C = P U$, where $P$ is the permutation matrix of the grade bijection and $U$ is upper triangular with diagonal entries 1 or −1. Equivalently, the grade bijection is the unique permutation part of the Bruhat decomposition of the Coxeter matrix. Along the way the paper proves that the grade bijection coincides with the classical bijection between indecomposable injective and projective modules given by syzygies, and uses the factorization to show that the permanent of any Auslander regular Coxeter matrix is 1 or −1, and to characterize distributive lattices by the pivot pattern of their Coxeter matrix.
Load-bearing premise
The argument assumes a known structural property of the grade bijection, cited from the literature rather than proved here: in the module $\mathrm{Ext}^r_A(S,A)$, every simple composition factor other than the socle has grade greater than $r$, and dually every top except one has cograde greater than $r$.
Editorial extensions
If this is right
- The permanent of the Coxeter matrix of any Auslander regular algebra is either 1 or −1, providing a fast necessary test: a permanent outside this set shows the algebra is not Auslander regular.
- For a finite lattice ordered by a linear extension, distributivity is equivalent to the condition that the first nonzero entry of each row of the Coxeter matrix lies in a different column; in that case the Coxeter permutation is the rowmotion bijection.
- For blocks of category O, the set of grades of modules equals twice the values of Lusztig's a-function.
- For an Auslander regular algebra with a simple-preserving duality, the Cartan matrix is symmetric and the grade bijection is the identity.
- The grade bijection of an Auslander-Gorenstein algebra equals the classical injective-to-projective syzygy bijection, so the two historical definitions describe the same map.
Reading between the lines
- Beyond the paper: because a Bruhat permutation is computed by pivoting, the result should make grade bijections computable in polynomial time for algebras where injective resolutions are slow; an implementation on quiver algebras would be a direct test.
- Beyond the paper: the ±1 permanent constraint is a necessary condition that may be much easier to verify than Auslander regularity; applying it to families of algebras of finite global dimension with unknown homological dimension could identify new candidates for Auslander regular algebras.
- Beyond the paper: the lattice theorem suggests a purely matrix-theoretic route to rowmotion on distributive lattices; one could test whether the entries of $U_2$ in the factorization carry additional order information about the lattice.
Formalized claims in Lean
-
Claim #1: The central claim is that for a finite-dimensional Auslander regular algebra, the grade bijection—the map sending a simple module to the top of the socle of its first nonzero Ext against the algebra—is encoded verbatim by linear algebra. When the simple modules are ordered so that grades increase with index, the Coxeter matrix factorizes as $C = P U$, where $P$ is the permutation matrix of the gra
/-- @claim 1 The central claim is that for a finite-dimensional Auslander regular algebra, the grade bijection—the map sending a simple module to the top of the socle of its first nonzero Ext against the algebra—is encoded verbatim by linear algebra. When the simple modules are ordered so that grades increase with index, the Coxeter matrix factorizes as $C = P U$, where $P$ is the permutation matrix of the gra -/ def central_claim : Prop :=
Editorial analysis
A structured set of objections, weighed in public.
Referee Report
Summary. The paper establishes that for Auslander-Gorenstein algebras, Iyama's grade bijection coincides with the Auslander-Reiten bijection, and for Auslander regular algebras with an admissible ordering of simples, the Coxeter matrix admits a Bruhat decomposition C = P U2 in which P is the permutation matrix of the grade bijection and U2 is upper triangular with diagonal entries ±1. From this it derives that the permanent of the Coxeter matrix is ±1. In a second part, it gives a characterisation of finite distributive lattices in terms of the Bruhat decomposition of their incidence algebra's Coxeter matrix, and applies the results to blocks of category O.
Significance. The linear-algebraic reinterpretation of the grade bijection is elegant and has immediate computational value, and the permanent corollary gives a fast numerical necessary condition for Auslander regularity. The lattice characterisation is striking and connects rowmotion to Coxeter matrices. The proof strategy is mostly clean: Theorem 3.3's equivalence and the AR/grades bijection proof are well structured, and the matrix argument for Theorem 3.13 is transparent once its hypotheses are granted. The main caveat is that the proof of Theorem 3.13 delegates a structurally essential point to a fact quoted from inside the proof of Iyama's Theorem 2.10 rather than to a stated theorem.
major comments (4)
- [Proposition 3.9 / Theorem 3.13] The proof of Theorem 3.13 rests on Proposition 3.9, which invokes "part (ii) of the proof of Theorem 2.10 in [I]" for two facts: all composition factors of Ext^r_A(S,A) other than the socle Dφ(S) have grade > r, and Dφ(S) occurs with multiplicity one. These facts are not stated in [I, Theorem 2.10] (as cited) and not proved here. The upper triangularity of φ^{-1}M in the proof of Theorem 3.13 depends on the first fact, and the assertion that the diagonal entries of U2 are ±1 depends on the second: if the multiplicity of Dφ(S) were m > 1, the corresponding diagonal entry would be ±m. Please provide a self-contained proof or a direct reference to a stated theorem containing these properties.
- [Lemma 4.1] The dot-product rule in the proof of Lemma 4.1 reads "C_{x·} · [P(y)] is −1 if y ≥ x and 0 otherwise," but no convention for the dimension vectors of projectives and injectives in the incidence algebra is specified. In particular, with Cartan entries e_{i,j} = dim e_j A e_i, the identity C·dim P(y)=-dim I(y) determines the direction of the inequality, and the subsequent equivalence "for y ∈ I, this is the same as −1 if y ≥ x ∨ z" depends on that direction. Since Corollary 4.2 and Theorem 4.9 rely on Lemma 4.1, please state the convention explicitly and verify the proof of Lemma 4.1 under that convention.
- [Lemma 4.5] The verification of the Coxeter matrix formula is given as a few dot-product assertions ("the dot product is therefore zero, as desired"). This is load-bearing for Theorems 4.7 and 4.9, which identify leftmost nonzero entries of rows via Δ(Y) and χ_red(Δ(Y)). Please expand the proof to show explicitly how the entries follow from the definition C = -ω^T ω^{-1} and the Möbius inversion for the incidence algebra, and state the precise support conventions for projective and injective dimension vectors.
- [Theorem 4.7] The statement "the leftmost non-zero entry in row Y is in column ⟨min Y^c⟩ = row^{-1}(Y)" needs clarification: the standard rowmotion formula row(I) = ⟨min(P \ I)⟩ gives ⟨min Y^c⟩ = row(Y), not row^{-1}(Y). The inverse appears because Y is a set of meet-irreducibles and the correspondence with R reverses order, but this is not spelled out. Please state the convention explicitly and define the rowmotion used on R and on order ideals of meet-irreducibles, so that the conclusion that the Coxeter permutation is the grade bijection/rowmotion is verifiable.
minor comments (5)
- [Title page] The running title contains a typo: "ma trices" should be "matrices".
- [Section 2.3.4] The phrase "where is acts as ν" should read "where it acts as ν".
- [Proof of Theorem 3.13] The matrix M is introduced as containing "multiplicity (with sign)"; please specify the sign convention, since the diagonal entries of U2 depend on it.
- [Example 3.15] The poset example would be easier to verify if a Hasse diagram were included.
- [Section 2.1] The notation "idim" is used without definition; it should be defined where first used, parallel to "pdim".
Circularity Check
No significant circularity; the main Bruhat-factorization theorem is derived, not assumed, and self-citations are background only.
full rationale
The central result, Theorem 3.13, is not circular. The matrix M is defined directly from the multiplicities of indecomposable injectives in the minimal injective coresolutions of projectives; the identity M = -C follows from the Cartan/Coxeter definitions, and the argument that phi^{-1}M is upper triangular uses only the admissible ordering and the external grade-bijection property cograde(phi(S)) = grade(S). No parameter is fitted, no prediction is renamed as an input, and the conclusion C = P U2 is not assumed. Theorem 3.3 is a genuine characterization proved from Lemma 3.2 and standard facts, not from the target equality. Proposition 3.9 does cite part (ii) of the proof of Iyama's Theorem 2.10 for two structural facts, including the multiplicity-one property of D phi(S); this is an external published result rather than a self-citation, and while it is a load-bearing dependency for the +/-1 diagonal, depending on external theorems is not circularity. The self-citations [IM] and [MTY] are used for background and for the rowmotion remark, but Theorem 4.7 independently proves that the Coxeter permutation for a distributive lattice is the rowmotion bijection using Lemma 4.5, so the self-citation is not needed for the main claim. No equation in the paper reduces to its own input by construction, and no fitted quantity is relabeled as a prediction.
Assumptions & free parameters
assumptions (6)
- standard math Iyama's grade bijection properties (cograde(phi(S)) = grade(S); composition series structure of Ext^r(S,A) in [I, Theorem 2.10])
- standard math Auslander-Reiten bijection results, especially [AR, Prop 5.4(b)] giving the bijection Omega^{-r} between projectives of injective dimension r and injectives of projective dimension r in r-Gorenstein algebras
- standard math Fundamental theorem of finite distributive lattices and the structure lemma for lattices with all proper upper intervals distributive (Lemma 4.3)
- standard math Bruhat decomposition existence and uniqueness of the permutation matrix for invertible matrices (Theorem 2.6)
- standard math Morita equivalence reduction to quiver algebras over algebraically closed fields
- standard math Auslander-Gorenstein algebras are Iwanaga-Gorenstein and the property is left-right symmetric
Cite this review
Pith. "Pith review of Auslander regular algebras and Coxeter matrices." pith.science (2026). https://pith.science/paper/ENSAKFIM
@misc{pith2026250109447,
author = {Pith},
title = {Pith review of: Auslander regular algebras and Coxeter matrices},
year = {2026},
howpublished = {\url{https://pith.science/paper/ENSAKFIM}},
note = {Machine review of arXiv:2501.09447}
}
abstract
We show that Iyama's grade bijection for Auslander-Gorenstein algebras coincides with the bijection introduced by Auslander-Reiten. This result uses a new characterisation of Auslander-Gorenstein algebras. Furthermore, we show that the grade bijection of an Auslander regular algebra coincides with the permutation matrix P in the Bruhat factorisation of the Coxeter matrix. This gives a new, purely linear algebraic interpretation of the grade bijection and allows us to calculate it in a much quicker way than was previously known. We give several applications of our main results. First, we show that the permanent of the Coxeter matrix of an Auslander regular algebra is either 1 or -1. Second, we obtain a new combinatorial characterisation of distributive lattices among the class of finite lattices. Explicitly, a lattice is distributive if and only if its Coxeter matrix can be written as PU where P is a permutation matrix and U is an upper triangular matrix. Other applications include new homological results about modules in blocks of category $\mathcal{O}$ of semisimple Lie algebras.
Forward citations
Cited by 3 Pith papers
-
Rowmotion and Echelonmotion
Echelonmotion, defined via the Bruhat decomposition of a poset's Cartan matrix, coincides with rowmotion on semidistributive lattices and on trim lattices under vertebral linear extensions, and echelon-independent lat...
-
A Serre-type criterion for $n$-Gorenstein rings and its application to Nakayama algebras
Over Auslander-Gorenstein Nakayama algebras, Ext^n(S,A) for odd n is always zero or simple, settling the Klász–Kleinau–Marczinzik conjecture via a (G_n) criterion and syzygy filtration.
-
A survey on Auslander-Gorenstein algebras
A survey of finite-dimensional Auslander-Gorenstein algebras, including classifications for monomial and incidence algebras and the identification of the Auslander-Reiten permutation with rowmotion, Ringel's homologic...
Reference graph
Works this paper leans on
-
[1]
Aigner, M.: Combinatorial theory. Classics in Mathematics. Berlin: Springer. x, 484 p. (1997)
work page 1997
-
[2]
Assem, I.; Simson, D.; Skowronski, A.: Elements of the representation theory of associative algebras. Vol. 1: Techniques of representation theory. London Mathematical Society Student Texts 65. Cambridge: Cambridge University Press, 458 p. (2006)
work page 2006
-
[3]
O.: Representation theory of Artin algebras
Auslander, M.; Reiten, I.; Smalo, S. O.: Representation theory of Artin algebras. Cambridge Studies in Advanced Mathematics, Volume 36, Cambridge University Press, 1997
work page 1997
-
[4]
Auslander, M., Reiten, I.: k-Gorenstein algebras and syzygy modules. J. Pure Appl. Algebra, 92, 1-27 (1994)
work page 1994
-
[5]
Proceedings of the American Mathematical Society Vol
Auslander, M., Reiten, I.: On a Generalized Version of the Nakayama Conjecture. Proceedings of the American Mathematical Society Vol. 52, No. 1 (1975), pp. 69-74
work page 1975
-
[6]
Bass, H.: On the ubiquity of Gorenstein rings. Math. Z. 82, 8-28 (1963)
work page 1963
-
[7]
Beilinson, A.; Ginzburg, V.; Soergel, W.: Koszul duality patterns in representation theory. J. Amer. Math. Soc. 9 (1996), no. 2, 473-527
work page 1996
-
[8]
Cambridge Studies in Advanced Mathematics, Volume 30, Cambridge University Press, 1991
Benson, D.: Representations and cohomology I: Basic representation theory of finite groups and associative algebras. Cambridge Studies in Advanced Mathematics, Volume 30, Cambridge University Press, 1991
work page 1991
Show all 38 references
-
[9]
Graduate Texts in Mathematics 225
Bump, D.: Lie groups. Graduate Texts in Mathematics 225. New York, NY: Springer. (2013)
2013
-
[10]
https://arxiv.org/abs/2012.11927
Chan, A.; Darp\"o, E.; Iyama, O.; Marczinzik, R.: Periodic trivial extension algebras and fractionally Calabi-Yau algebras. https://arxiv.org/abs/2012.11927
2012 arXiv
-
[11]
https://arxiv.org/abs/1712.04587
Chen, X.: Gorenstein Homological Algebra of Artin Algebras. https://arxiv.org/abs/1712.04587
-
[12]
Eilenberg, S.: Algebras of cohomologically finite dimension. Comment. Math. Helv. 28 (1954), no.1, 310--319
1954
-
[13]
Fuller, K.; Zimmermann-Huisgen, B.: On the generalized Nakayama conjecture and the Cartan determinant problem. Trans. Am. Math. Soc. 294, 679-691 (1986)
1986
-
[14]
Grant, J.: The Nakayama automorphism of a self-injective preprojective algebra. Bull. Lond. Math. Soc. 52, No. 1, 137-152 (2020)
2020
-
[15]
Humphreys, J. E. Representations of semisimple Lie algebras in the BGG category O . Graduate Studies in Mathematics 94. Providence, RI: American Mathematical Society (AMS) 289 p. (2008)
2008
-
[16]
2, 528-535
Iyama, O.: Symmetry and duality on n -Gorenstein rings , Journal of Algebra, 269 (2003), no. 2, 528-535
2003
-
[17]
Iyama, O.: Higher-dimensional Auslander-Reiten theory on maximal orthogonal subcategories. Adv. Math. 210, No. 1, 22-50 (2007)
2007
-
[18]
Iyama, O.: The relationship between homological properties and representation theoretic realization of artin algebras. Trans. Amer. Math. Soc. 357 (2005), no. 2, 709-734
2005
-
[19]
Advances in Mathematics Volume 398, 2022
Iyama, O.; Marczinzik, R.: Distributive lattices and Auslander regular algebras. Advances in Mathematics Volume 398, 2022
2022
-
[20]
In progress
Kl\'asz, V.: The Auslander-Gorenstein condition for monomial algebras. In progress
-
[21]
Ko, H.; Mazorchuk, V.; Mrden, R.: Some homological properties of category O . VI. Doc. Math. 26, 1237-1269 (2021)
2021
-
[22]
A.: Spectral analysis of finite dimensional algebras and singularities
Lenzing, H.; de la Pena, J. A.: Spectral analysis of finite dimensional algebras and singularities. Trends in representation theory of algebras and related topics. Proceedings of the 12th international conference on representations of algebras and workshop. EMS Series of Congr...
2008
-
[23]
Fields Institute Communications
Madsen, D.: Projective dimensions and Nakayama algebras. Fields Institute Communications. 45. Amer. Math. Soc., Providence, RI, 2005. 247-265
2005
-
[24]
Marczinzik, R.; Thomas, H.; Yildirim, E.: On the interaction of the Coxeter transformation and the rowmotion bijection. J. Combinatorial Algebra 8 (2024) no. 3--4, 359--374
2024
-
[25]
Mazorchuk, V.: Some homological properties of the category O . II. Represent. Theory 14 (2010), 249-263
2010
-
[26]
Encyclopedia of Mathematics and its Applications 6
Minc, H.: Permanents. Encyclopedia of Mathematics and its Applications 6. Cambridge: Cambridge University Press. 205 p. (2011)
2011
-
[27]
H.; Olesky, D
Odeh, O. H.; Olesky, D. D.; van den Driessche, P.: Bruhat decomposition and numerical stability. SIAM J. Matrix Anal. Appl. 19, No. 1, 89-98 (1998)
1998
-
[28]
J.; Shakiban, C.: Applied linear algebra
Olver, P. J.; Shakiban, C.: Applied linear algebra. Undergraduate Texts in Mathematics. Springer. xxv, 679 p. (2018)
2018
-
[29]
The QPA-team, QPA - Quivers, path algebras and representations - a GAP package, Version 1.33; 2022 https: //folk.ntnu.no/oyvinso/QPA/
2022
-
[30]
M.: The finitistic dimension of a Nakayama algebra
Ringel, C. M.: The finitistic dimension of a Nakayama algebra. Journal of Algebra, Volume 576, 2021, Pages 95-145, ISSN 0021-8693
2021
-
[31]
Stein et al., Sage Mathematics Software, The Sage Development Team, 2022, http: //www.sagemath.org
A. Stein et al., Sage Mathematics Software, The Sage Development Team, 2022, http: //www.sagemath.org
2022
-
[32]
Gordon and Breach Science Publishers, Algebra, Logic and Applications, Volume 4, 1992
Simson, D.: Linear representations of partially ordered sets and vector space categories. Gordon and Breach Science Publishers, Algebra, Logic and Applications, Volume 4, 1992
1992
-
[33]
I: Basic representation theory
Skowronski, A.; Yamagata, K.: Frobenius algebras. I: Basic representation theory. EMS Textbooks in Mathematics. Z\"urich: European Mathematical Society (2011)
2011
-
[34]
Notices Amer
Striker, J.: Dynamical algebraic combinatorics: promotion, rowmotion, and resonance. Notices Amer. Math. Soc. 64 (2017), no. 6, 543-549
2017
-
[35]
Thomas, H.; Williams, N.: Rowmotion in slow motion. Proc. Lond. Math. Soc. (3) 119 (2019), no. 5, 1149-1178
2019
-
[36]
N.; Bau, D
Trefethen, L. N.; Bau, D. III: Numerical linear algebra. Philadelphia, PA: SIAM, Society for Industrial and Applied Mathematics. xii, 361 p. (1997)
1997
-
[37]
Pure and Applied Mathematics 232
Van Oystaeyen, F.: Algebraic geometry for associative algebras. Pure and Applied Mathematics 232. New York, NY: Marcel Dekker. vi, 287 p. (2000)
2000
-
[38]
Journal of Algebra Volume 82, Issue 2, June 1983, Pages 353-357
Zacharia, D.: On the Cartan matrix of an artin algebra of global dimension two. Journal of Algebra Volume 82, Issue 2, June 1983, Pages 353-357
1983
Reviewed August 10, 2026 · model on record in the stance chip above.
Discussion (0). Continue with ORCID to comment.