Parallel Algorithms for Group Isomorphism via Code Equivalence
Pith reviewed 2026-05-10 11:45 UTC · model grok-4.3
The pith
Isomorphism testing for coprime abelian extensions and certain central-radical groups is in AC³.
A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.
Core claim
We exhibit AC³ isomorphism tests for coprime extensions H ⋉ N where H is elementary Abelian and N is Abelian; and groups where Rad(G) = Z(G) is elementary Abelian and G = Soc*(G). The polynomial-time isomorphism tests for both of these families crucially leveraged small instances of Linear Code Equivalence. Here, we combine Luks' group-theoretic method for Graph Isomorphism with the fact that G is given by its multiplication table, to implement the corresponding instances of Linear Code Equivalence in AC³. As a byproduct, isomorphism testing of arbitrary central-radical groups is decidable using AC circuits of depth O(log³ n) and size n^{O(log log n)}.
What carries the argument
Small instances of linear code equivalence, solved in AC³ by applying Luks' group-theoretic techniques directly to the multiplication table presentation of the input groups.
If this is right
- Isomorphism testing for the two specified families of groups lies in AC³.
- Isomorphism testing for arbitrary central-radical groups lies in AC circuits of depth O(log³ n) and size n^{O(log log n)}.
- The earlier polynomial-time algorithms for these families are strengthened to constant-depth parallel algorithms.
- The reductions from group isomorphism to linear code equivalence can be realized inside AC³ when the group is presented by a multiplication table.
Where Pith is reading between the lines
- The same reduction-plus-Luks approach may yield AC³ or low-depth AC algorithms for additional families of groups whose isomorphism reduces to code equivalence.
- Multiplication-table presentations appear especially well-suited for extracting high parallelism from algebraic decision problems.
- The result supplies a concrete route to study the parallel complexity of graph isomorphism through its group-theoretic formulation.
Load-bearing premise
The input groups are given by their complete multiplication tables, which supplies direct access to the group operation inside the circuit.
What would settle it
An explicit pair of groups from one of the families whose isomorphism status cannot be correctly decided by any family of AC³ circuits built from the derived code-equivalence instances.
read the original abstract
In this paper, we exhibit $\textsf{AC}^{3}$ isomorphism tests for coprime extensions $H \ltimes N$ where $H$ is elementary Abelian and $N$ is Abelian; and groups where $\text{Rad}(G) = Z(G)$ is elementary Abelian and $G = \text{Soc}^{*}(G)$. The fact that isomorphism testing for these families is in $\textsf{P}$ was established respectively by Qiao, Sarma, and Tang (STACS 2011), and Grochow and Qiao (CCC 2014, SIAM J. Comput. 2017). The polynomial-time isomorphism tests for both of these families crucially leveraged small (size $O(\log |G|)$) instances of Linear Code Equivalence (Babai, SODA 2011). Here, we combine Luks' group-theoretic method for Graph Isomorphism (FOCS 1980, J. Comput. Syst. Sci. 1982) with the fact that $G$ is given by its multiplication table, to implement the corresponding instances of Linear Code Equivalence in $\textsf{AC}^{3}$. As a byproduct of our work, we show that isomorphism testing of arbitrary central-radical groups is decidable using $\textsf{AC}$ circuits of depth $O(\log^3 n)$ and size $n^{O(\log \log n)}$. This improves upon the previous bound of $n^{O(\log \log n)}$-time due to Grochow and Qiao (ibid.).
Editorial analysis
A structured set of objections, weighed in public.
Referee Report
Summary. In this paper, the authors exhibit AC³ isomorphism tests for coprime extensions H ⋉ N where H is elementary Abelian and N is Abelian, as well as for groups where Rad(G) = Z(G) is elementary Abelian and G = Soc*(G). They achieve this by implementing small instances of Linear Code Equivalence in AC³ circuits using Luks' group-theoretic method combined with the multiplication-table model of the groups. As a byproduct, they provide an AC circuit algorithm of depth O(log³ n) and size n^{O(log log n)} for isomorphism testing of arbitrary central-radical groups, improving upon previous time bounds.
Significance. If the central claims hold, this work is significant for advancing the parallel complexity theory of group isomorphism. It provides the first AC³ algorithms for these specific group families by parallelizing the code equivalence subroutines. The improvement to AC circuits with polylog depth for central-radical groups is a clear strengthening of prior results. The paper credits the polynomial-time foundations from Qiao-Sarma-Tang and Grochow-Qiao, and focuses on the parallel implementation, which is a valuable contribution in the field.
major comments (1)
- [the description of the AC³ implementation for Linear Code Equivalence (via Luks' method)] The claim that the small (O(log |G|)) Linear Code Equivalence instances arising in the isomorphism tests for the two families can be solved in AC³ via Luks' method depends on a non-trivial parallelization that exploits the multiplication table to avoid full enumeration. The manuscript must explicitly verify that computing stabilizers, testing conjugacy, and constructing the required linear transformations introduces no extra logarithmic depth layer or implicit sequential search, as any such step would violate the depth-3 bound even if the algorithm remains polynomial-time.
minor comments (1)
- [Abstract] The abstract and introduction should more explicitly contrast the new AC³ bound with the prior polynomial-time algorithms of Qiao-Sarma-Tang and Grochow-Qiao, including a brief remark on why the multiplication-table presentation is essential for the circuit construction.
Simulated Author's Rebuttal
We thank the referee for their positive assessment of the significance of our results and for the constructive major comment. We address the point regarding explicit verification of the AC³ implementation below.
read point-by-point responses
-
Referee: The claim that the small (O(log |G|)) Linear Code Equivalence instances arising in the isomorphism tests for the two families can be solved in AC³ via Luks' method depends on a non-trivial parallelization that exploits the multiplication table to avoid full enumeration. The manuscript must explicitly verify that computing stabilizers, testing conjugacy, and constructing the required linear transformations introduces no extra logarithmic depth layer or implicit sequential search, as any such step would violate the depth-3 bound even if the algorithm remains polynomial-time.
Authors: We agree that the manuscript would benefit from an explicit depth analysis to confirm the AC³ bound. In the revised version we will add a dedicated subsection detailing the parallelization. Because the Linear Code Equivalence instances have size O(log |G|), the multiplication-table representation permits constant-depth element access. Stabilizer and conjugacy computations are performed via parallel orbit-stabilizer algorithms whose depth is O(log log n) for these small instances; constructing the linear transformations reduces to parallel linear algebra over GF(q), which lies in AC¹. These factors compose with the existing O(log³ n) bound without introducing an additional logarithmic layer, keeping the overall algorithm in AC³. We will include the corresponding circuit-depth calculations and pseudocode. revision: yes
Circularity Check
No circularity: new AC^3 implementation of known small-code-equivalence instances
full rationale
The derivation rests on external prior results (Qiao-Sarma-Tang STACS 2011 and Grochow-Qiao CCC 2014) that already placed the two families in P via small Linear Code Equivalence instances, plus the external Luks 1980/1982 group-theoretic technique. The paper's contribution is an independent circuit implementation that exploits the given multiplication table to achieve depth 3; this step does not reduce to a self-definition, a fitted parameter renamed as prediction, or a load-bearing self-citation. The byproduct AC bound for central-radical groups is likewise an improvement on a prior external time bound rather than a re-derivation of the input. No equations or claims in the provided text exhibit the forbidden patterns.
Axiom & Free-Parameter Ledger
axioms (2)
- domain assumption Groups are given by multiplication tables
- standard math Luks' group-theoretic method for Graph Isomorphism applies to the relevant code-equivalence instances
Reference graph
Works this paper leans on
-
[1]
[AK06] V. Arvind and Piyush P. Kurur. Graph isomorphism is in SPP.Information and Computation, 204(5):835–852, 2006.doi:10.1016/j.ic.2006.02.002. [Bab16] L´ aszl´ o Babai. Graph isomorphism in quasipolynomial time [extended abstract]. InSTOC’16— Proceedings of the 48th Annual ACM SIGACT Symposium on Theory of Computing, pages 684–697. ACM, New York,
-
[2]
Preprint of full version at arXiv:1512.03547v2 [cs.DS].doi: 10.1145/2897518.2897542. [Bar92] D.A.M. Barrington. Quasipolynomial size circuit classes. In[1992] Proceedings of the Seventh Annual Structure in Complexity Theory Conference, pages 86–93, 1992.doi:10.1109/SCT. 1992.215383. [BB99] L´ aszl´ o Babai and Robert Beals. A polynomial-time theory of bla...
-
[3]
[BCP82] L Babai, P.J Cameron, and P.P P´ alfy
SIAM.doi: 10.1137/1.9781611973082.107. [BCP82] L Babai, P.J Cameron, and P.P P´ alfy. On the orders of primitive groups with restricted nonabelian composition factors.Journal of Algebra, 79(1):161–168, 1982.doi:10.1016/ 0021-8693(82)90323-4. [BCQ12] L´ aszl´ o Babai, Paolo Codenotti, and Youming Qiao. Polynomial-time isomorphism test for groups with no ab...
-
[4]
Superpolynomial circuits, almost sparse oracles and the exponential hierarchy
[BH92] Harry Buhrman and Steven Homer. Superpolynomial circuits, almost sparse oracles and the exponential hierarchy. In R. K. Shyamasundar, editor,Foundations of Software Technology and Theoretical Computer Science, 12th Conference, New Delhi, India, December 18-20, 1992, Proceedings, volume 652 ofLecture Notes in Computer Science, pages 116–127. Springer,
work page 1992
-
[5]
doi:10.1007/3-540-56287-7\_99. [BKLM01] David A. Mix Barrington, Peter Kadau, Klaus-J¨ orn Lange, and Pierre McKenzie. On the complexity of some problems on groups input as multiplication tables.J. Comput. Syst. Sci., 63(2):186–200, 2001.doi:10.1006/jcss.2001.1764. 26 [BLS87] L. Babai, E. Luks, and A. Seress. Permutation groups in NC. InSTOC 1987, STOC ’8...
-
[6]
Association for Computing Machinery.doi:10.1145/ 28395.28439. [BMW17] Peter A. Brooksbank, Joshua Maglione, and James B. Wilson. A fast isomorphism test for groups whose Lie algebra has genus 2.Journal of Algebra, 473:545–590, 2017.doi:10.1016/j. jalgebra.2016.12.007. [BQ12] L´ aszl´ o Babai and Youming Qiao. Polynomial-time isomorphism test for groups wi...
work page doi:10.1016/j 2017
-
[7]
[Bra23] Jendrik Brachter.Combinatorial approaches to the group isomorphism problem
Springer LNCS 6651, 2012.doi:10.4230/ LIPIcs.STACS.2012.453. [Bra23] Jendrik Brachter.Combinatorial approaches to the group isomorphism problem. PhD thesis, TU Darmstadt, Sept
work page 2012
-
[8]
[BS84] L´ aszl´ o Babai and Endre Szemer´ edi
URL:https://tuprints.ulb.tu-darmstadt.de/26387/1/thesis_ brachter.pdf. [BS84] L´ aszl´ o Babai and Endre Szemer´ edi. On the complexity of matrix group problems I. In25th Annual Symposium on Foundations of Computer Science, West Palm Beach, Florida, USA, 24-26 October 1984, pages 229–240. IEEE Computer Society, 1984.doi:10.1109/SFCS.1984. 715919. [BS20] J...
-
[9]
Prelim- inary version in Proceedings of the 2024 International Symposium on Symbolic and Algebraic Computation, ISSAC 2024 (DOI:10.1145/3666000.3669691).doi:10.46298/theoretics.25
work page doi:10.1145/3666000.3669691).doi:10.46298/theoretics.25 2024
-
[10]
[CH03] John J. Cannon and Derek F. Holt. Automorphism group computation and isomorphism testing in finite groups.J. Symb. Comput., 35:241–267, March 2003.doi:10.1016/S0747-7171(02) 00133-5. [CL24] Nathaniel A. Collins and Michael Levet. Count-free Weisfeiler–Leman and group isomor- phism.International Journal of Algebra and Computation, 34(03):283–330, 20...
-
[11]
Report TR10-117.doi:10.1145/2540088
Prelimi- nary version appeared in FSTTCS ’10; ECCC Tech. Report TR10-117.doi:10.1145/2540088. [DM96] John D. Dixon and Brian Mortimer.Permutation groups, volume 163 ofGraduate Texts in Mathematics. Springer-Verlag, New York, 1996.doi:10.1007/978-1-4612-0731-3. 27 [DT24] Bireswar Das and Dhara Thakkar. The minimal faithful permutation degree of groups with...
-
[12]
[DW22] Heiko Dietrich and James B
Association for Computing Machinery.doi:10.1145/3618260.3649641. [DW22] Heiko Dietrich and James B. Wilson. Group isomorphism is nearly-linear time for most orders. In2021 IEEE 62nd Annual Symposium on Foundations of Computer Science (FOCS), pages 457–467, 2022.doi:10.1109/FOCS52979.2021.00053. [Ebe91] Wayne Eberly. Decomposition of algebras over finite f...
-
[13]
Association for Computing Machinery.doi:10.1145/2591796.2591865. [ELGO02] Bettina Eick, C. R. Leedham-Green, and E. A. O’Brien. Constructing automorphism groups of p-groups.Comm. Algebra, 30(5):2271–2295, 2002.doi:10.1081/AGB-120003468. [ES17] Michael Elberfeld and Pascal Schweitzer. Canonizing graphs of bounded tree width in logspace. ACM Trans. Comput. ...
-
[14]
Springer International Publishing.doi:10.1007/978-3-030-39951-1_6. [FN70] V. Felsch and J. Neub¨ user. On a programme for the determination of the automorphism group of a finite group. InComputational Problems in Abstract Algebra (Proc. Conf., Oxford, 1967), pages 59–60
-
[15]
[FSS84] Merrick L. Furst, James B. Saxe, and Michael Sipser. Parity, circuits, and the polynomial-time hierarchy.Math. Syst. Theory, 17(1):13–27, 1984.doi:10.1007/BF01744431. [GJL25] Joshua A. Grochow, Dan Johnson, and Michael Levet. Complexity of identifying Fitting-free groups. In Artur Jez and Jan Otop, editors,Fundamentals of Computation Theory - 25th...
-
[17]
doi:10.46298/lmcs-21(4:20)2025
Pre- liminary version appeared in the proceedings of GandALF 2023 (DOI:10.4204/EPTCS.390.12). doi:10.46298/lmcs-21(4:20)2025. [GL26] Joshua A. Grochow and Michael Levet. On the parallel complexity of group isomorphism via Weisfeiler–Leman.Journal of Computer and System Sciences, 156:103703,
-
[18]
Preliminary version appeared in the proceedings of FCT 2023, DOI:10.1007/978-3-031-43587-4 17.doi: 10.1016/j.jcss.2025.103703. [GQ15] Joshua A. Grochow and Youming Qiao. Polynomial-time isomorphism test of groups that are tame extensions - (extended abstract). InAlgorithms and Computation - 26th International Symposium, ISAAC 2015, Nagoya, Japan, December...
-
[19]
Also available as arXiv:1309.1776 [cs.DS] and ECCC Technical Report TR13-123.doi:10.1137/15M1009767
Preliminary version in IEEE Con- ference on Computational Complexity (CCC) 2014 (DOI:10.1109/CCC.2014.19). Also available as arXiv:1309.1776 [cs.DS] and ECCC Technical Report TR13-123.doi:10.1137/15M1009767. 28 [HBD17] Harald Andr´ es Helfgott, Jitendra Bajpai, and Daniele Dona. Graph isomorphisms in quasi- polynomial time, 2017.doi:10.48550/ARXIV.1710.04...
-
[20]
17 Klaus Jansen and Kim-Manuel Klein
[IPZ01] Russell Impagliazzo, Ramamohan Paturi, and Francis Zane. Which problems have strongly exponential complexity?Journal of Computer and System Sciences, 63(4):512–530, 2001.doi: 10.1006/jcss.2001.1774. [IQ19] G´ abor Ivanyos and Youming Qiao. Algorithms based on *-algebras, and their applications to isomorphism of polynomials with one secret, group i...
-
[21]
Preprint of full version at arXiv:2508.06478 [cs.DS].doi:10.1007/978-3-032-22469-9\_11
Springer Nature Switzerland. Preprint of full version at arXiv:2508.06478 [cs.DS].doi:10.1007/978-3-032-22469-9\_11. [KL90] William M. Kantor and Eugene M. Luks. Computing in quotient groups. In Harriet Ortiz, editor,Proceedings of the 22nd Annual ACM Symposium on Theory of Computing, May 13-17, 1990, Baltimore, Maryland, USA, pages 524–534. ACM, 1990.doi...
-
[22]
Canonizing Graphs of Bounded Rank- Width in Parallel via Weisfeiler-Leman
[LRS24] Michael Levet, Puck Rombach, and Nicholas Sieger. Canonizing Graphs of Bounded Rank- Width in Parallel via Weisfeiler-Leman. In Hans L. Bodlaender, editor,19th Scandinavian Symposium and Workshops on Algorithm Theory (SWAT 2024), volume 294 ofLeibniz Interna- tional Proceedings in Informatics (LIPIcs), pages 32:1–32:18, Dagstuhl, Germany,
work page 2024
-
[23]
[LST25] Michael Levet, Pranjal Srivastava, and Dhara Thakkar
Schloss Dagstuhl – Leibniz-Zentrum f¨ ur Informatik.doi:10.4230/LIPIcs.SWAT.2024.32. [LST25] Michael Levet, Pranjal Srivastava, and Dhara Thakkar. Complexity of minimal faithful per- mutation degree for fitting-free groups. In Artur Jez and Jan Otop, editors,Fundamen- tals of Computation Theory - 25th International Symposium, FCT 2025, Wroc law, Poland, S...
-
[24]
Complexity of Constructing Minimal Faithful Permutation Representations for Fitting-free Groups
Preprint of full version at arXiv:2501.16039v3 [cs.DS].doi: 10.1007/978-3-032-04700-7\_26. [LSZ77] R. J. Lipton, L. Snyder, and Y. Zalcstein. The complexity of word and isomorphism problems for finite groups. Yale University Dept. of Computer Science Research Report # 91,
work page internal anchor Pith review Pith/arXiv arXiv doi:10.1007/978-3-032-04700-7
-
[25]
URL: https://apps.dtic.mil/dtic/tr/fulltext/u2/a053246.pdf. [Luk82] Eugene M. Luks. Isomorphism of graphs of bounded valence can be tested in polynomial time. Journal of Computer and System Sciences, 25(1):42–65, 1982.doi:10.1016/0022-0000(82) 90009-5. [Luk93] Eugene Luks. Permutation groups and polynomial-time computation.DIMACS Series in Dis- crete Math...
-
[26]
29 [Luk15] Eugene M. Luks. Group isomorphism with fixed subnormal chains, 2015.arXiv:1511.00151. [LW12] Mark L. Lewis and James B. Wilson. Isomorphism in expanding families of indistinguishable groups. 4(1):73–110, 2012.doi:10.1515/gcc-2012-0008. [MC83] Pierre McKenzie and Stephen A. Cook. The parallel complexity of the abelian permutation group membershi...
-
[27]
Association for Computing Machinery.doi:10.1145/800133.804331. [Mul87] Ketan Mulmuley. A fast parallel algorithm to compute the rank of a matrix over an arbitrary field.Comb., 7(1):101–104, 1987.doi:10.1007/BF02579205. [QST11] Youming Qiao, Jayalal M. N. Sarma, and Bangsheng Tang. On isomorphism testing of groups with normal Hall subgroups. InProc. 28th S...
- [28]
-
[29]
[Sch88] Uwe Sch¨ oning. Graph isomorphism is in the low hierarchy.Journal of Computer and System Sciences, 37(3):312 – 323, 1988.doi:10.1016/0022-0000(88)90010-4. [Ser77] Jean-Pierre Serre.Linear representations of finite groups, volume Vol. 42 ofGraduate Texts in Mathematics. Springer-Verlag, New York-Heidelberg, french edition, 1977.doi:10.1007/ 978-1-4...
-
[30]
[Tau55] D. R. Taunt. Remarks on the isomorphism problem in theories of construction of finite groups. Mathematical Proceedings of the Cambridge Philosophical Society, 51(1):16–24, 1955.doi:10. 1017/S030500410002987X. [Tor04] Jacobo Tor´ an. On the hardness of graph isomorphism.SIAM J. Comput., 33(5):1093–1108, 2004.doi:10.1137/S009753970241096X. [Vol99] H...
-
[31]
URL:https://oparu.uni-ulm.de/xmlui/bitstream/handle/ 123456789/3923/vts_7264_10267.pdf. 30 [Wil19] James B. Wilson. The threshold for subgroup profiles to agree is logarithmic.Theory of Com- puting, 15(19):1–25, 2019.doi:10.4086/toc.2019.v015a019. [Wol94] Marty J. Wolf. Nondeterministic circuits, space complexity and quasigroups.Theoretical Com- puter Sci...
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