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Discrete Subgroups of Lie Groups

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Discrete subgroups of Lie groups Michael Kapovich (University of California, Davis), Gregory Margulis (Yale University), Gregory Soifer (Bar Ilan University), Dave Witte Morris (University of Lethbridge) December 9, 2019–December 13, 2019 1 Overview of the Field Recent years have seen a great deal of progress in our understanding of “thin” subgroups, which are dis- crete matrix groups that have infinite covolume in their Zariski closure. (The subgroups of finite covolume are called “lattices” and, generally speaking, are much better understood.) Traditionally, thin subgroups are required to be contained in arithmetic lattices, which is natural in the context of number-theoretic and al- gorithmic problems but, from the geometric or dynamical viewpoint, is not necessary. Thin subgroups have deep connections with number theory (see e.g. [6, 7, 8, 18, 22]), geometry (e.g. [1, 14, 16, 38]), and dynamics (e.g. [3, 23, 27, 29]). The well-known “Tits Alternative” [44] (based on the classical “ping-pong argument” of Felix Klein) constructs free subgroups of any matrix group that is not virtually solvable. (In most cases, it is easy to arrange that the resulting free group is thin.) Sharpening and refining this classical construction is a very active and fruitful area of research that has settled numerous old problems. For instance, Breuillard and Gelander [11] proved a quantitative form of the Tits Alternative, which shows that the generators of a free subgroup can be chosen to have small word length, with respect to any generating set of the ambient group. Kapovich, Leeb and Porti [26] provided a coarse-geometric proof of the existence of free subgroups that are Anosov. In a somewhat different vein, Margulis and Soifer [32] proved that SL(n; Z); n ≥ 4; contains 2 free products of the form Z ?Fk (where Fk is the free group of rank k) for all k ≥ 1. (Answering a question of Platonov and Prasad, this implies that SL(n; Z) has maximal subgroups of infinite index that are not free groups.) Also, it has been shown that SL(n; Z) contains Coxeter groups (and, hence, right-angled Artin groups) when n is sufficiently large. Ping-pong type constructions have also emerged as an important technical tool in other contexts, such as for disproving the invariable generation property (by constructing a thin subgroup that intersects every conjugacy class [19]) and for proving the expander property for Cayley graphs of finite quotients of thin groups [22]. Conversely, there are also obstructions to the existence of thin subgroups. For example, no thin, Zariski- dense subgroup of SL(n; Z) contains SL(3; Z) [45]. Similarly, it has been shown in certain situations that thin, discrete, Zariski-dense subgroups cannot contain a lattice in a maximal unipotent subgroup of the ambi- ent group [5, 35, 46]. The study of certain natural (and properly discontinuous) actions of thin groups is another important line of research. The famous Auslander Conjecture concerns actions of thin groups on affine spaces. (Namely, it is conjectured that if a group acts properly discontinuously and cocompactly on an affine space Rn, then the group is virtually solvable.) Actions on more general geometric spaces (such as flag varieties) are also 1 2 important. While it seems that nothing of interest can be said about such actions in general, a great deal of progress has been made in recent years analyzing actions of thin subgroups that satisfy further restrictions. The Anosov property has been of particular interest (see, for example, [23], [27], and [49]), as well as other forms of strengthening of discreteness, e.g. regularity, which has its origin in [3], see also [27]. This conference brought together a range of specialists whose expertise in order to educate each other in the broad spectrum of techniques and problems in the theory of discrete subgroups of Lie groups. A number of young researchers, including graduate students, actively participated in the conference as well. 2 Open Problems On the first day of the workshop we conducted a problem session in the theory of discrete subgroups of Lie groups. Below are problems collected during and after the workshop. 2.1 Proper actions on affine spaces Let Γ be a finitely generated group, ρ :Γ ! GL(n; R) be the given representation, Z1(Γ; ρ) be the space of ρ-cocycles with values in Rn, i.e. maps n u :Γ ! R ; u(αβ) = u(α) + ρ(α)u(β); α; β 2 Γ: Note that Z1(Γ; ρ) is a finite-dimensional real vector space. Each cocycle u 2 Z1(Γ; ρ) determines an affine n action ρu of Γ on V = R : ρu(γ): x 7! ρ(γ)x + u(γ): 1 n Let C = Cρ ⊂ Z (Γ; ρ) denote the subset consisting of cocycles such that the action ρu of Γ on R is properly discontinuous. Question 2.1 (N. Tholozan). Is C open in Z1(Γ; ρ)? Is it convex? Note that the answer is positive for n = 3, this follows from the results of [21]. Question 2.2 (N. Tholozan). Does Cρ depend continuously on ρ? Here one has to be careful with the topology used on the set of subsets of Z1(Γ; ρ). For closed subsets one uses Shabauty topology. For instance, if the subsets C are open, their complements are closed and, hence, one can interpret the question as of the continuity of the complement with respect to ρ. For a finitely generated group Γ, let P (Γ; n) denote the subset of Hom(Γ; Aff(Rn)) consisting of rep- resentations defining proper actions of Γ on Rn. Let PA(Γ; n) ⊂ P (Γ; n) denote the subset consisting of actions ρu with P -Anosov linear part ρ (for some parabolic subgroup P < GL(n; R)). Question 2.3 (G. Soifer). To which extent P (Γ; n) is open? Note that in general P (Γ; n) is not open even for n = 3, for instance, one can take a rank 2 free group Γ and a representation ρu 2 P (Γ; 3) whose linear part ρ contains unipotent elements. A small perturbation of ρu will yield a representation (with linear part in SO(2; 1)) with nondiscrete linear part, hence, a non-proper affine action. One can also perturb a representation so that the linear part is deformed to a Zariski dense subgroup of SL(3; R), again resulting in a non-proper action. It is known that 3 PA(Γ; 3) \ Hom(Γ; SO(2; 1) n R ) is open in 3 Hom(Γ; SO(2; 1) n R ) (this follows from the results of [21] and stability of Anosov representations). Question 2.4 (G. Soifer). To which extent P (Γ; n) is open in general? 3 Conjecture 2.5 (The Auslander conjecture). If Γ < Aff(Rn) is a subgroup which acts properly discontinu- ously and co-compactly on Rn, then Γ is virtually solvable. Abels, Margulis and Soifer proved the Auslander conjecture for the dimensions n ≤ 6 and observed that the following problem is important for the further progress towards the Auslander conjecture: Conjecture 2.6 (Abels, Margulis, Soifer). Let Γ < O(4; 3) n R7 < Aff(R7) be a subgroup acting properly discontinuously and co-compactly on R7. Then the linear part of Γ is not Zariski dense in SO(4; 3). Question 2.7. Does there exist a properly discontinuous cocompact group of affine transformations isomor- phic to the fundamental group of a closed hyperbolic manifold? Partial progress towards a negative answer to this problem was described at the talk by Suhyoung Choi at the workshop: He proved that such an action cannot exist under certain assumptions on its linear part (strengthening the P -Anosov condition). Question 2.8 (G. Mostow). Suppose that M = Rn=Γ is an affine manifold, such that the linear part of Γ is n in SLn(R). Thus, Γ preserves the standard volume form on R and, hence, M has a canonical volume form as well. Does V ol(M) < 1 imply that M is compact? This question is motivated by Mostow’s theorem that lattices in solvable groups are cocompact. Question 2.9 (G. Soifer). Study subgroups Γ < Aff(Cn) acting properly discontinuously on Cn. More specifically: Question 2.10 (G. Soifer). Does there exist a free nonabelian subgroup Γ < Aff(C3) acting properly discon- 3 3 tinuously on C ? For instance, consider the adjoint action of SL2(C) on C (identified with the Lie algebra of SL2(C)). Consider a generic representation ρ : F2 = ha; bi ! SL(2; C). Does there exist a cocycle 1 3 3 m m u 2 Z (F2; C ) and m > 0 such that the action on C of ha ; b i given by ρu is properly discontinuous? Conjecture 2.11 (Markus Conjecture). Suppose that M is a compact n-dimensional affine manifold whose linear holonomy is in SL(n; R). Is M complete? This conjecture is known when the linear holonomy of M has “discompacity 1” (Y. Carriere, [13]), e.g. when M is a flat Lorentzian manifold, and also for convex affine manifolds of dimension ≤ 5, [25]. Conjecture 2.12 (M. Kapovich). Suppose that M is a compact n-dimensional affine manifold whose linear holonomy is contained in a rank one subgroup of SL(n; R). Is M complete? 2.2 Discrete subgroups of SL(3; R) Question 2.13 (M. Kapovich). 1. Does there exist a discrete a subgroup Γ < SL(3; R) isomorphic to Z2 ? Z and containing only regular diagonalizable elements? 2. Does there exist a discrete subgroup Γ < SL(3; Z) isomorphic to Z2 ? Z? Note that there are known examples, [43], of discrete subgroups Γ < SL(3; R) isomorphic to Z2?Z where Z2 is super-singular: It is generated by three singular diagonalizable matrices A; B; C satisfying ABC = 1.
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