scholarly journals Statistics of subgroups of the modular group

2021 ◽  
pp. 1-61
Author(s):  
Frédérique Bassino ◽  
Cyril Nicaud ◽  
Pascal Weil
Keyword(s):  
Finite Index ◽  
Modular Group ◽  
Large Deviation ◽  
Free Subgroup ◽  
Index Formula ◽  
Index Subgroup ◽  
Finitely Generated ◽  
Finite Index Subgroup ◽  
Free Subgroups ◽  
Finite Index Subgroups

We count the finitely generated subgroups of the modular group [Formula: see text]. More precisely, each such subgroup [Formula: see text] can be represented by its Stallings graph [Formula: see text], we consider the number of vertices of [Formula: see text] to be the size of [Formula: see text] and we count the subgroups of size [Formula: see text]. Since an index [Formula: see text] subgroup has size [Formula: see text], our results generalize the known results on the enumeration of the finite index subgroups of [Formula: see text]. We give asymptotic equivalents for the number of finitely generated subgroups of [Formula: see text], as well as of the number of finite index subgroups, free subgroups and free finite index subgroups. We also give the expected value of the isomorphism type of a size [Formula: see text] subgroup and prove a large deviation statement concerning this value. Similar results are proved for finite index and for free subgroups. Finally, we show how to efficiently generate uniformly at random a size [Formula: see text] subgroup (respectively, finite index subgroup, free subgroup) of [Formula: see text].

scholarly journals VIRTUAL ALGEBRAIC FIBRATIONS OF KÄHLER GROUPS

2019 ◽  
pp. 1-19
Author(s):  
STEFAN FRIEDL ◽  
STEFANO VIDUSSI
Keyword(s):  
Fundamental Group ◽  
Finite Index ◽  
Surface Groups ◽  
Index Subgroup ◽  
Finitely Generated ◽  
Kähler Surfaces ◽  
Kähler Groups ◽  
Strong Relation ◽  
Finite Index Subgroup ◽  
Finite Index Subgroups

This paper stems from the observation (arising from work of Delzant) that “most” Kähler groups $G$ virtually algebraically fiber, that is, admit a finite index subgroup that maps onto $\mathbb{Z}$ with finitely generated kernel. For the remaining ones, the Albanese dimension of all finite index subgroups is at most one, that is, they have virtual Albanese dimension $va(G)\leqslant 1$ . We show that the existence of algebraic fibrations has implications in the study of coherence and higher BNSR invariants of the fundamental group of aspherical Kähler surfaces. The class of Kähler groups with $va(G)=1$ includes virtual surface groups. Further examples exist; nonetheless, they exhibit a strong relation with surface groups. In fact, we show that the Green–Lazarsfeld sets of groups with $va(G)=1$ (virtually) coincide with those of surface groups, and furthermore that the only virtually RFRS groups with $va(G)=1$ are virtually surface groups.

ON KAZHDAN CONSTANTS OF FINITE INDEX SUBGROUPS IN SLn(ℤ)

2012 ◽  
Vol 22 (03) ◽  
pp. 1250026
Author(s):  
UZY HADAD
Keyword(s):  
Finite Index ◽  
Congruence Subgroup ◽  
Infinite Family ◽  
Principal Congruence ◽  
The Other ◽  
Index Subgroup ◽  
Generating Set ◽  
Principal Congruence Subgroup ◽  
Finite Index Subgroup ◽  
Finite Index Subgroups

We prove that for any finite index subgroup Γ in SL n(ℤ), there exists k = k(n) ∈ ℕ, ϵ = ϵ(Γ) > 0, and an infinite family of finite index subgroups in Γ with a Kazhdan constant greater than ϵ with respect to a generating set of order k. On the other hand, we prove that for any finite index subgroup Γ of SL n(ℤ), and for any ϵ > 0 and k ∈ ℕ, there exists a finite index subgroup Γ′ ≤ Γ such that the Kazhdan constant of any finite index subgroup in Γ′ is less than ϵ, with respect to any generating set of order k. In addition, we prove that the Kazhdan constant of the principal congruence subgroup Γn(m), with respect to a generating set consisting of elementary matrices (and their conjugates), is greater than [Formula: see text], where c > 0 depends only on n. For a fixed n, this bound is asymptotically best possible.

scholarly journals Congruence testing for odd subgroups of the modular group

2014 ◽  
Vol 17 (1) ◽  
pp. 206-208
Author(s):  
Thomas Hamilton ◽  
David Loeffler
Keyword(s):  
Finite Index ◽  
Modular Group ◽  
Congruence Subgroup ◽  
Index Subgroup ◽  
Finite Index Subgroup

AbstractWe give a computationally effective criterion for determining whether a finite-index subgroup of $\mathrm{SL}_2(\mathbf{Z})$ is a congruence subgroup, extending earlier work of Hsu for subgroups of $\mathrm{PSL}_2(\mathbf{Z})$.

scholarly journals Virtual Retraction Properties in Groups

2019 ◽  
Author(s):  
Ashot Minasyan
Keyword(s):  
Finite Index ◽  
Wreath Products ◽  
Graph Products ◽  
Index Subgroup ◽  
Finitely Generated ◽  
Simple Criterion ◽  
Cyclic Subgroups ◽  
Group A ◽  
Finite Index Subgroup ◽  
Virtually Free Group

Abstract If $G$ is a group, a virtual retract of $G$ is a subgroup, which is a retract of a finite index subgroup. Most of the paper focuses on two group properties: property (LR), that all finitely generated subgroups are virtual retracts; and property (VRC), that all cyclic subgroups are virtual retracts. We study the permanence of these properties under commensurability, amalgams over retracts, graph products, and wreath products. In particular, we show that (VRC) is stable under passing to finite index overgroups, while (LR) is not. The question whether all finitely generated virtually free groups satisfy (LR) motivates the remaining part of the paper, studying virtual free factors of such groups. We give a simple criterion characterizing when a finitely generated subgroup of a virtually free group is a free factor of a finite index subgroup. We apply this criterion to settle a conjecture of Brunner and Burns.

scholarly journals APPLICATIONS OF p-DEFICIENCY AND p-LARGENESS

2011 ◽  
Vol 21 (04) ◽  
pp. 547-574 ◽  
Author(s):  
J. O. BUTTON ◽  
A. THILLAISUNDARAM
Keyword(s):  
Finite Index ◽  
Coxeter Groups ◽  
Index Subgroup ◽  
Finitely Generated ◽  
P Deficiency ◽  
Finite Presentation ◽  
Finite Index Subgroup ◽  
Finitely Presented

We use Schlage-Puchta's concept of p-deficiency and Lackenby's property of p-largeness to show that a group having a finite presentation with p-deficiency greater than 1 is large, which implies that Schlage-Puchta's infinite finitely generated p-groups are not finitely presented. We also show that for all primes p at least 7, any group having a presentation of p-deficiency greater than 1 is Golod–Shafarevich, and has a finite index subgroup which is Golod–Shafarevich for the remaining primes. We also generalize a result of Grigorchuk on Coxeter groups to odd primes.

scholarly journals Groups with positive rank gradient and their actions

2018 ◽  
Vol 68 (2) ◽  
pp. 353-360
Author(s):  
Mark Shusterman
Keyword(s):  
Finite Index ◽  
Free Groups ◽  
Infinite Index ◽  
Index Subgroup ◽  
Profinite Groups ◽  
Finitely Generated ◽  
Finite Product ◽  
Rank Gradient ◽  
Finite Index Subgroup ◽  
Positive Rank

Abstract We show that given a finitely generated LERF group G with positive rank gradient, and finitely generated subgroups A, B ≤ G of infinite index, one can find a finite index subgroup B0 of B such that [G : 〈A ∪ B0〉] = ∞. This generalizes a theorem of Olshanskii on free groups. We conclude that a finite product of finitely generated subgroups of infinite index does not cover G. We construct a transitive virtually faithful action of G such that the orbits of finitely generated subgroups of infinite index are finite. Some of the results extend to profinite groups with positive rank gradient.

scholarly journals IRREDUCIBLE COXETER GROUPS

2007 ◽  
Vol 17 (03) ◽  
pp. 427-447 ◽  
Author(s):  
LUIS PARIS
Keyword(s):  
Direct Product ◽  
Finite Index ◽  
Coxeter Group ◽  
Coxeter Groups ◽  
Index Subgroup ◽  
Central Automorphism ◽  
Finite Coxeter Group ◽  
Irreducible Components ◽  
Finite Index Subgroup ◽  
Finite Index Subgroups

We prove that a non-spherical irreducible Coxeter group is (directly) indecomposable and that an indefinite irreducible Coxeter group is strongly indecomposable in the sense that all its finite index subgroups are (directly) indecomposable. Let W be a Coxeter group. Write W = WX1 × ⋯ × WXb × WZ3, where WX1, … , WXb are non-spherical irreducible Coxeter groups and WZ3 is a finite one. By a classical result, known as the Krull–Remak–Schmidt theorem, the group WZ3 has a decomposition WZ3 = H1 × ⋯ × Hq as a direct product of indecomposable groups, which is unique up to a central automorphism and a permutation of the factors. Now, W = WX1 × ⋯ × WXb × H1 × ⋯ × Hq is a decomposition of W as a direct product of indecomposable subgroups. We prove that such a decomposition is unique up to a central automorphism and a permutation of the factors. Write W = WX1 × ⋯ × WXa × WZ2 × WZ3, where WX1, … , WXa are indefinite irreducible Coxeter groups, WZ2 is an affine Coxeter group whose irreducible components are all infinite, and WZ3 is a finite Coxeter group. The group WZ2 contains a finite index subgroup R isomorphic to ℤd, where d = |Z2| - b + a and b - a is the number of irreducible components of WZ2. Choose d copies R1, … , Rd of ℤ such that R = R1 × ⋯ × Rd. Then G = WX1 × ⋯ × WXa × R1 × ⋯ × Rd is a virtual decomposition of W as a direct product of strongly indecomposable subgroups. We prove that such a virtual decomposition is unique up to commensurability and a permutation of the factors.

scholarly journals From local to global conjugacy of subgroups of relatively hyperbolic groups

2017 ◽  
Vol 27 (03) ◽  
pp. 299-314
Author(s):  
Oleg Bogopolski ◽  
Kai-Uwe Bux
Keyword(s):  
Finite Index ◽  
Minimal Length ◽  
Hyperbolic Groups ◽  
Index Subgroup ◽  
Finitely Generated ◽  
Limit Group ◽  
Relatively Hyperbolic Groups ◽  
Finitely Generated Group ◽  
Finite Index Subgroup ◽  
Relatively Hyperbolic

Suppose that a finitely generated group [Formula: see text] is hyperbolic relative to a collection of subgroups [Formula: see text]. Let [Formula: see text] be subgroups of [Formula: see text] such that [Formula: see text] is relatively quasiconvex with respect to [Formula: see text] and [Formula: see text] is not parabolic. Suppose that [Formula: see text] is elementwise conjugate into [Formula: see text]. Then there exists a finite index subgroup of [Formula: see text] which is conjugate into [Formula: see text]. The minimal length of the conjugator can be estimated. In the case, where [Formula: see text] is a limit group, it is sufficient to assume only that [Formula: see text] is a finitely generated and [Formula: see text] is an arbitrary subgroup of [Formula: see text].

Greenberg's Theorem for Quasiconvex Subgroups of Word Hyperbolic Groups

1996 ◽  
Vol 48 (6) ◽  
pp. 1224-1244 ◽  
Author(s):  
Ilya Kapovich ◽  
Hamish Short
Keyword(s):  
Finite Index ◽  
Free Groups ◽  
Hyperbolic Group ◽  
Hyperbolic Groups ◽  
Index Subgroup ◽  
Finitely Generated ◽  
Word Hyperbolic Group ◽  
Quasiconvex Subgroups ◽  
Word Hyperbolic Groups ◽  
Finite Index Subgroup

AbstractAnalogues of a theorem of Greenberg about finitely generated subgroups of free groups are proved for quasiconvex subgroups of word hyperbolic groups. It is shown that a quasiconvex subgroup of a word hyperbolic group is a finite index subgroup of only finitely many other subgroups.

scholarly journals Isomorphisms Between Big Mapping Class Groups

2018 ◽  
Vol 2020 (10) ◽  
pp. 3084-3099 ◽  
Author(s):  
Juliette Bavard ◽  
Spencer Dowdall ◽  
Kasra Rafi
Keyword(s):  
Finite Index ◽  
Outer Automorphism ◽  
Mapping Class Groups ◽  
Mapping Class ◽  
Index Subgroup ◽  
Class Groups ◽  
Infinite Type ◽  
Finite Index Subgroup ◽  
Finite Index Subgroups ◽  
Orientable Surfaces

Abstract We show that any isomorphism between mapping class groups of orientable infinite-type surfaces is induced by a homeomorphism between the surfaces. Our argument additionally applies to automorphisms between finite-index subgroups of these “big” mapping class groups and shows that each finite-index subgroup has finite outer automorphism group. As a key ingredient, we prove that all simplicial automorphisms between curve complexes of infinite-type orientable surfaces are induced by homeomorphisms.

Sign in / Sign up

Export Citation Format

Share Document