Conjugacy in Abstract Semigroups, Transformation and Diagram Monoids, and Conjugacy Growth
Pith reviewed 2026-05-24 10:39 UTC · model grok-4.3
The pith
The four-equation conjugacy relation cf n admits complete class classifications in the full transformation monoid T_n and the symmetric inverse monoid I_n.
A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.
Core claim
The relation a cf n b holds when there exist g, h in S^1 such that ag = gb, bh = ha, hag = b, and gbh = a; this yields exhaustive enumerations of equivalence classes in T_n, I_n, and related monoids, together with structural results on natural conjugacy in diagram semigroups and an exact growth estimate in polycyclic monoids.
What carries the argument
The cf n relation, the four-equation definition that generates equivalence classes whose structure is classified by direct combinatorial arguments in the listed monoids.
If this is right
- Every cf n-class in T_n can be listed by rank and image type.
- The same enumeration applies without change to I_n and to endomorphism monoids of finite G-sets.
- Natural conjugacy on partition monoids, Brauer monoids and partial Brauer monoids is completely determined by the same four equations.
- The conjugacy growth function of any polycyclic monoid admits an exact asymptotic formula.
Where Pith is reading between the lines
- The four-equation definition may serve as a uniform test for conjugacy across other families of finite monoids not treated in the paper.
- The classification techniques could be adapted to decide cf n-membership algorithmically for monoids given by presentations.
- Growth estimates obtained for polycyclic monoids suggest that similar asymptotics might exist for related monoids with solvable word problems.
- The interplay results with standard conjugacy relations indicate that cf n could refine existing invariants used in automata theory.
Load-bearing premise
The four equations produce equivalence classes whose complete listing in T_n, I_n and the diagram monoids requires no additional hidden constraints or missed case distinctions.
What would settle it
An explicit pair of elements in T_3 whose membership in the same cf n-class contradicts the listed classification tables, or a pair that satisfies cf n but violates one of the four defining equations.
Figures
read the original abstract
We study conjugacy relations on semigroups and monoids, focusing on the relation $a \cfn b$, defined by the existence of $g,h \in S^1$ such that $ag = gb$, $bh = ha$, $hag = b$, and $gbh = a$. This notion emerged as one that yields particularly elegant results. The interplay between $\cfn$ and other standard conjugacy relations is analyzed, and some results on special classes of abstract semigroups are established. We then specialize to the case of transformation semigroups. A complete classification of $\cfn$-classes is obtained for the full transformation monoid $\mathcal{T}_n$, the symmetric inverse monoid $\mathcal{I}_n$, and the endomorphism monoid of $G$-sets, among others. We also investigate the natural conjugacy in diagram semigroups, including the partition monoid, the Brauer monoid, and the partial Brauer monoid. Finally, we investigate the conjugacy growth function in polycyclic monoids and obtain a precise asymptotic estimate. The paper concludes with some open problems.
Editorial analysis
A structured set of objections, weighed in public.
Referee Report
Summary. The paper introduces the conjugacy relation a cf_n b on a semigroup S, defined by the existence of g, h in S^1 satisfying the four equations ag = gb, bh = ha, hag = b, and gbh = a. It analyzes the relation's interplay with other conjugacy notions in abstract semigroups, then specializes to obtain a complete classification of cf_n-classes in the full transformation monoid T_n, the symmetric inverse monoid I_n, and the endomorphism monoid of G-sets. The work further studies natural conjugacy in diagram semigroups (partition, Brauer, and partial Brauer monoids) and derives a precise asymptotic estimate for the conjugacy growth function in polycyclic monoids, concluding with open problems.
Significance. If the classifications are exhaustive, the paper would advance semigroup theory by exhibiting a conjugacy relation that produces clean structural results in key monoids and by supplying explicit descriptions and growth asymptotics that can serve as benchmarks for further work on conjugacy invariants.
major comments (2)
- [Section on transformation semigroups (classification for T_n and I_n)] The central claim of a complete classification of cf_n-classes in T_n (and similarly I_n) rests on exhaustive enumeration via the four-equation definition; the case analysis by rank, image, and kernel must be shown to cover every configuration without omitted cases or additional constraints imposed by the equations hag = b and gbh = a that would alter the partition into classes.
- [Section on diagram semigroups] For the diagram monoids, the investigation of natural conjugacy should explicitly verify that the four equations reduce to the expected combinatorial conditions on diagrams without introducing extra equivalences not captured by the standard diagrammatic description.
minor comments (2)
- [Introduction] The abstract states that cf_n 'emerged as one that yields particularly elegant results'; a brief comparison table or explicit statement of which prior relations (e.g., ~_L, ~_R, ~_J) it refines would help readers situate the new relation.
- [Section on polycyclic monoids] Notation for the polycyclic monoids and the precise statement of the asymptotic estimate for the conjugacy growth function should be cross-referenced to the relevant theorem number for quick lookup.
Simulated Author's Rebuttal
We thank the referee for their careful reading and constructive comments on our manuscript. We address the major comments point by point below, and we will revise the manuscript to incorporate explicit verifications as suggested.
read point-by-point responses
-
Referee: [Section on transformation semigroups (classification for T_n and I_n)] The central claim of a complete classification of cf_n-classes in T_n (and similarly I_n) rests on exhaustive enumeration via the four-equation definition; the case analysis by rank, image, and kernel must be shown to cover every configuration without omitted cases or additional constraints imposed by the equations hag = b and gbh = a that would alter the partition into classes.
Authors: The manuscript presents a complete classification through exhaustive case analysis on the rank, image, and kernel of the transformations in T_n and I_n. We have ensured that the four equations are fully accounted for in determining the equivalence classes. To address the referee's concern and make the exhaustiveness explicit, we will add a new lemma that systematically verifies that all possible configurations are covered and that the equations hag = b and gbh = a do not impose additional constraints beyond those used in the classification. This revision will be included in the updated version of the paper. revision: yes
-
Referee: [Section on diagram semigroups] For the diagram monoids, the investigation of natural conjugacy should explicitly verify that the four equations reduce to the expected combinatorial conditions on diagrams without introducing extra equivalences not captured by the standard diagrammatic description.
Authors: In the section on diagram semigroups, the natural conjugacy is defined and analyzed by reducing the four equations to conditions on the partitions and connections represented by the diagrams. We believe this reduction is accurate and does not introduce extra equivalences. However, to provide the explicit verification requested, we will add a short proposition in the revised manuscript that derives the combinatorial conditions directly from the equations, confirming they match the expected description without additional relations. revision: yes
Circularity Check
No circularity: direct classification from explicit four-equation definition
full rationale
The paper introduces the cf_n relation via an explicit four-equation definition (ag=gb, bh=ha, hag=b, gbh=a) and proceeds by direct case analysis on the combinatorial structure of the target monoids (T_n, I_n, diagram monoids, etc.). No parameters are fitted to data, no predictions are renamed from inputs, and no load-bearing steps reduce to self-citations or prior ansatzes by the same authors. The completeness claim rests on exhaustive enumeration within the given relation, which is an independent mathematical argument rather than a definitional tautology. This is the normal case of a self-contained classification theorem.
Axiom & Free-Parameter Ledger
axioms (1)
- standard math A semigroup is a set equipped with an associative binary operation.
Reference graph
Works this paper leans on
-
[1]
J. Andr´ e, J. Ara´ ujo, P.J. Cameron, The classification of partition homogeneous groups with applications to semigroup theory, J. Algebra 452 (2016), 288–310
work page 2016
-
[2]
Y. Antol ´ ın and L. Ciobanu, Formal conjugacy growth in acylindr ically hyperbolic groups, Int. Math. Res. Not. IMRN 2017 (2017), 121–157
work page 2017
-
[3]
J. Ara´ ujo, Normal semigroups of endomorphisms of proper ind ependence algebras are idempotent gen- erated, Proc. Edinb. Math. Soc. (2) 45 (2002), 205–217
work page 2002
-
[4]
J. Ara´ ujo, J.P. Ara´ ujo, P.J. Cameron, T. Dobson, A. Hulpke, and P. Lopes, Imprimitive permutations in primitive groups, J. Algebra 486 (2017), 396–416
work page 2017
-
[5]
J. Ara´ ujo, J.P. Ara´ ujo, W. Bentz, P.J. Cameron, and P. Spiga, A transversal property for permutation groups motivated by partial transformations, J. Algebra 573 (2021), 741–759
work page 2021
-
[6]
J. Ara´ ujo, W. Bentz, and P.J. Cameron, Groups synchronizing a transformation of non-uniform kernel, Theoret. Comput. Sci. 498 (2013), 1–9
work page 2013
-
[7]
J. Ara´ ujo, W. Bentz, and P.J. Cameron, Orbits of primitive k-ho mogenous groups on (n − k)-partitions with applications to semigroups, Trans. Amer. Math. Soc. 371 (2019), 105–136
work page 2019
-
[8]
J. Ara´ ujo, W. Bentz, and P.J. Cameron, The existential trans versal property: a generalization of ho- mogeneity and its impact on semigroups, Trans. Amer. Math. Soc. 374 (2021), 1155–1195
work page 2021
-
[9]
J. Ara´ ujo, W. Bentz, and P.J. Cameron, Primitive permutation g roups and strongly factorizable trans- formation semigroups, J. Algebra 565 (2021), 513–530. 74
work page 2021
-
[10]
J. Ara´ ujo, W. Bentz, P.J. Cameron, M. Kinyon, and J. Koniecz ny, Matrix theory for independence algebras, Linear Algebra Appl. 642 (2022), 221–250
work page 2022
-
[11]
J. Ara´ ujo, W. Bentz, P.J. Cameron, G. Royle, and A. Schaefe r, Primitive groups, graph endomorphisms and synchronization, Proc. Lond. Math. Soc. (3) 113 (2016), 829–867
work page 2016
-
[12]
J. Ara´ ujo, W. Bentz, and J. Konieczny, The largest subsemila ttices of the endomorphism monoid of an independence algebra, Linear Algebra Appl. 458 (2014), 60–79
work page 2014
-
[13]
J. Ara´ ujo and P.J. Cameron, Primitive groups synchronize non -uniform maps of extreme ranks, J. Combin. Theory Ser. B 106 (2014), 98–114
work page 2014
-
[14]
J. Ara´ ujo and P.J. Cameron, Two generalizations of homogene ity in groups with applications to regular semigroups, Trans. Amer. Math. Soc. 368 (2016), 1159–1188
work page 2016
-
[15]
J. Ara´ ujo, P.J. Cameron, J.D. Mitchell, and M. Neunh¨ offer, The classification of normalizing groups, J. Algebra 373 (2013), 481–490
work page 2013
-
[16]
J. Ara´ ujo, P.J. Cameron, and B. Steinberg, Between primitive and 2-transitive: synchronization and its friends, EMS Surv. Math. Sci. 4 (2017), 101–184
work page 2017
-
[17]
J. Ara´ ujo, M. Edmundo, and S. Givant, v∗ -algebras, independence algebras and logic, Internat. J. Algebra Comput. 21 (2011), 1237–1257
work page 2011
-
[18]
J. Ara´ ujo and J. Fountain, The origins of independence algebras , World Scientific Publishing Co., Inc., River Edge, NJ, 2004, 54–67
work page 2004
-
[19]
J. Ara´ ujo, M. Kinyon, and J. Konieczny, Conjugacy in inverse semigroups, J. Algebra , 533 (2019), 142–173
work page 2019
-
[20]
J. Ara´ ujo, M. Kinyon, J. Konieczny, and A. Malheiro, Four notions of conjugacy for abstract semigroups, Proc. Roy. Soc. Edinburgh Sect. A 147 (2017), 1169–1214
work page 2017
-
[21]
J. Ara´ ujo, M. Kinyon, J. Konieczny, and A. Malheiro, Decidabilit y and independence of conjugacy problems in finitely presented monoids, Theoret. Comput. Sci. 731 (2018), 88–98
work page 2018
-
[22]
J. Ara´ ujo and J. Konieczny, Automorphisms groups of centr alizers of idempotents, J. Algebra 269 (2003), 227–239
work page 2003
-
[23]
J. Ara´ ujo and J. Konieczny, Semigroups of transformations preserving an equivalence relation and a cross-section, Comm. Algebra 32 (2004), 1917–1935
work page 2004
-
[24]
J. Ara´ ujo and J. Konieczny, Automorphisms of endomorphism monoids of relatively free bands, Proc. Edinb. Math. Soc. 50 (2007), 1–21
work page 2007
-
[25]
J. Ara´ ujo and J. Konieczny, Centralizers in the full transfor mation semigroup, Semigroup Forum 86 (2013), 1–31
work page 2013
-
[26]
J. Ara´ ujo, J. Konieczny, and A. Malheiro, Conjugation in semig roups, J. Algebra 403 (2014), 93–134
work page 2014
-
[27]
J. Ara´ ujo and J.D. Mitchell, Relative ranks in the monoid of endomorphisms of an independence algebra, Monatsh. Math. 151 (2007), 1–10
work page 2007
-
[28]
J. Ara´ ujo, J.D. Mitchell, and C. Schneider, Groups that toget her with any transformation generate regular semigroups for idempotent generated semigroups, J. Algebra 343 (2011), 93–106
work page 2011
-
[29]
J. Ara´ ujo, J.M. Mitchell, and N. Silva, On generating countable s ets of endomorphisms, Alg. Univers. 50 (2003), 61–67. 75
work page 2003
-
[30]
J. Ara´ ujo and F. Wehrung, Embedding properties of endomor phism semigroups, Fund. Math. 202 (2009), 125–146
work page 2009
-
[31]
R. Bacher and P. de la Harpe, Conjugacy growth series of some infinitely generated groups, Int. Math. Res. Not. IMRN (2018), 1532–1584
work page 2018
-
[32]
H. Barcelo and A. Ram, Combinatorial representation theory, New perspectives in algebraic combina- torics (Berkeley, CA, 1996–97) , 23–90, Math. Sci. Res. Inst. Publ., 38, Cambridge University Pres s, Cambridge, 1999
work page 1996
-
[33]
F. Bergeron, G. Labelle, and P. Leroux, Combinatorial species and tree-like structures , Encyclopedia of Mathematics and its Applications 67, Cambridge University Press, Cambridge, 1998
work page 1998
-
[34]
Biggs, Algebraic Graph Theory , Cambridge University Press, Cambridge, 1993
N. Biggs, Algebraic Graph Theory , Cambridge University Press, Cambridge, 1993
work page 1993
-
[35]
E. Breuillard and Y. de Cornulier, On conjugacy growth for solva ble groups, Illinois J. Math. 54 (2010), 389–395
work page 2010
-
[36]
E. Breuillard, Y. Cornulier, A. Lubotzky, and C. Meiri, On conjug acy growth of linear groups, Math. Proc. Cambridge Philos. Soc. 154 (2013), 261–277
work page 2013
-
[37]
P.J. Cameron and C. Szab´ o, Independence algebras, J. London Math. Soc. (2) 61 (2000), 321–334
work page 2000
-
[38]
G. Casella and R.L. Berger, Statistical inference, Second edition, Duxbury Advanced Series, Thomson Learning, 2002
work page 2002
-
[39]
L. Ciobanu, A. Evetts, and M.-C. Ho, The conjugacy growth of the soluble Baumslag-Solitar groups, New York J. Math. 26 (2020), 473–495
work page 2020
-
[40]
L. Ciobanu and S. Hermiller, Conjugacy growth series and langua ges in groups, Trans. Amer. Math. Soc. 366 (2014), 2803–2825
work page 2014
-
[41]
L. Ciobanu, S. Hermiller, D. Holt, and S. Rees, Conjugacy langua ges in groups, Israel J. Math. 211 (2016), 311–347
work page 2016
-
[42]
L. Ciobanu, S. Hermiller, and V. Mercier, Formal conjugacy gro wth in graph products I, Groups Geom. Dyn. 17 (2023), 427–457
work page 2023
-
[43]
A.H. Clifford and G.B. Preston, The Algebraic Theory of Semigroups , Mathematical Surveys, No. 7, Amer. Math. Soc., Providence, Rhode Island, 1964 (Vol. I) and 196 7 (Vol. II)
work page 1964
-
[44]
Coornaert, Asymptotic growth of conjugacy classes in finit ely-generated free groups, Internat
M. Coornaert, Asymptotic growth of conjugacy classes in finit ely-generated free groups, Internat. J. Algebra Comput. 15 (2005), 887–892
work page 2005
-
[45]
M. Coornaert and G. Knieper, Growth of conjugacy classes in G romov hyperbolic groups, Geom. Funct. Anal. 12 (2002), 464–478
work page 2002
-
[46]
M. Coornaert and G. Knieper, An upper bound for the growth o f conjugacy classes in torsion-free word hyperbolic groups, Internat. J. Algebra Comput. 14 (2004), 395–401
work page 2004
-
[47]
I. Dolinka, J. East, A. Evangelou, D. FitzGerald, N. Ham, J. Hyd e, and N. Loughlin, Enumeration of idempotents in diagram semigroups and algebras, J. Combin. Theory Ser. A 131 (20015), 119–152
-
[48]
East, Presentations for (singular) partition monoids: a new approach, Math
J. East, Presentations for (singular) partition monoids: a new approach, Math. Proc. Cambridge Philos. Soc. 165 (2018), 549–562
work page 2018
-
[49]
J. East and R.D. Gray, Diagram monoids and Graham–Houghton g raphs: idempotents and generating sets of ideals, J. Combin. Theory Ser. A 146 (2017), 63–128. 76
work page 2017
-
[50]
J. East and R.D. Gray, Ehresmann theory and partition monoids , J. Algebra 579 (2021), 318–352
work page 2021
-
[51]
Evetts, Rational growth in virtually abelian groups, Illinois J
A. Evetts, Rational growth in virtually abelian groups, Illinois J. Math. 63 (2019), 513–549
work page 2019
-
[52]
Evetts, Conjugacy growth in the higher Heisenberg groups , Glasg
A. Evetts, Conjugacy growth in the higher Heisenberg groups , Glasg. Math. J. 65 (2023), 148–169
work page 2023
-
[53]
V.H. Fernandes, Semigroups of order preserving mappings on a finite chain: a new class of divisors, Semigroup Forum 54 (1997), 230–236
work page 1997
-
[54]
V.H. Fernandes, The monoid of all injective order preserving pa rtial transformations on a finite chain, Semigroup Forum 62 (2001), 178–204
work page 2001
-
[55]
V.H. Fernandes, Presentations for some monoids of partial tr ansformations on a finite chain: a survey, Semigroups, Algorithms, Automata and Languages, Coimbra, 2001, 363–378, World Sci. Publ., River Edge, NJ (2002)
work page 2001
-
[56]
Fink, Conjugacy growth and width of certain branch groups , Internat
E. Fink, Conjugacy growth and width of certain branch groups , Internat. J. Algebra Comput. 24 (2014), 1213–1231
work page 2014
-
[57]
FitzGerald and K.W Lau, On the partition monoid and some relat ed semigroups, Bull
D.G. FitzGerald and K.W Lau, On the partition monoid and some relat ed semigroups, Bull. Aust. Math. Soc. 83 (2011), 273–288
work page 2011
-
[58]
J. Fountain and V. Gould, Relatively free algebras with weak exch ange properties, J. Aust. Math. Soc. 75 (2003), 355–384
work page 2003
-
[59]
J. Fountain and V. Gould, Products of idempotent endomorphis ms of relatively free algebras with weak exchange properties, Proc. Edinb. Math. Soc. (2) 50 (2007), 343–362
work page 2007
-
[60]
J. Fountain and A. Lewin, Products of idempotent endomorphis ms of an independence algebra of finite rank, Proc. Edinb. Math. Soc. (2) 35 (1992), 493–500
work page 1992
-
[61]
J. Fountain and A. Lewin, Products of idempotent endomorphis ms of an independence algebra of infinite rank, Math. Proc. Cambridge Philos. Soc. 114 (1993), 303–319
work page 1993
-
[62]
W. Fulton and J. Harris, Representation Theory: A First Course , Springer-Verlag, New York, 1991
work page 1991
-
[63]
O. Ganyushkin and V. Mazorchuk, Classical finite transformation semigroups: an introducti on, Algebra and Applications 9, Springer-Verlag, London, 2010
work page 2010
-
[64]
J. Gordon and M. Liebeck, Representations and characters of groups , Cambridge University Press, New York, 2001
work page 2001
-
[65]
Gould, Independence algebras, Alg
V. Gould, Independence algebras, Alg. Univers. 33 (1995), 294–318
work page 1995
-
[66]
Gould, Independence algebras, basis algebras and semigrou ps of quotients, Proc
V. Gould, Independence algebras, basis algebras and semigrou ps of quotients, Proc. Edinb. Math. Soc. (2) 53 (2010), 697–729
work page 2010
-
[67]
J.J. Graham and G.I. Lehrer, Cellular algebras, Invent. Math. , 123 (1996), 1–34
work page 1996
-
[68]
A. Grau, The semigroup of endomorphisms with restricted rang e of an independence algebra, Semigroup Forum 106 (2023), 128–159
work page 2023
-
[69]
Gray, Idempotent rank in endomorphism monoids of finite inde pendence algebras, Proc
R. Gray, Idempotent rank in endomorphism monoids of finite inde pendence algebras, Proc. Roy. Soc. Edinb. Sect. A 137 (2007), 303–331
work page 2007
-
[70]
J.P.C. Greenlees and J.P. May, Equivariant stable homotopy theo ry, Handbook of algebraic topology , 277–323, North-Holland, Amsterdam, 1995
work page 1995
-
[71]
Guba and Sapir, On the conjugacy growth functions of grou ps, Illinois J
V. Guba and Sapir, On the conjugacy growth functions of grou ps, Illinois J. Math. 54 (2010), 301–313. 77
work page 2010
-
[72]
R.M. Guralnick, M. Larsen, and P.H. Tiep, Representation growt h in positive characteristic and conju- gacy classes of maximal subgroups, Duke Math. J. 161 (2012) 107–137
work page 2012
-
[73]
T. Halverson and A. Ram, Partition algebras, European J. Combin. 26 (2005), 869–921
work page 2005
-
[74]
R. Hartmann, A. Henke, S. Koenig, and R. Paget, Cohomologica l stratification of diagram algebras, Math. Ann. 347 (2010), 765–804
work page 2010
-
[75]
Howie, Fundamentals of semigroup theory , Oxford University Press, New York, 1995
J.M. Howie, Fundamentals of semigroup theory , Oxford University Press, New York, 1995
work page 1995
-
[76]
B. Hasselblatt and A. Katok, A first course in dynamics: with a panorama of recent developm ents, Cambridge University Press, New York, 2003
work page 2003
-
[77]
M. Hull and D. Osin, Conjugacy growth of finitely generated gro ups, Adv. Math. 235 (2013), 361–389
work page 2013
-
[78]
M. Hull and D. Osin, Corrigendum to Conjugacy growth of finitely generated groups , Adv. Math. 294 (2016), 857–859
work page 2016
-
[79]
Jack, On the complexity of inverse semigroup conjugacy, Semigroup Forum 106(2023), 618–632
T. Jack, On the complexity of inverse semigroup conjugacy, Semigroup Forum 106(2023), 618–632
work page 2023
-
[80]
M. Kinyon and D. Stanovsk´ y, Abelianness and centrality in inver se semigroups, https://urldefense.com/v3/__https://arxiv.org/abs/2209.13805__;!!NCZxaNi9jForCP_SxBKJCA!TvgakaLUEPBs
discussion (0)
Sign in with ORCID, Apple, or X to comment. Anyone can read and Pith papers without signing in.