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The integers with their usual topology are a discrete subgroup of the real numbers.

In mathematics, a topological group G is called a discrete group if there is no limit point in it (i.e., for each element in G, there is a neighborhood which only contains that element). Equivalently, the group G is discrete if and only if its identity is isolated.[1]

A subgroup H of a topological group G is a discrete subgroup if H is discrete when endowed with the subspace topology from G. In other words there is a neighbourhood of the identity in G containing no other element of H. For example, the integers, Z, form a discrete subgroup of the reals, R (with the standard metric topology), but the rational numbers, Q, do not.

Any group can be endowed with the discrete topology, making it a discrete topological group. Since every map from a discrete space is continuous, the topological homomorphisms between discrete groups are exactly the group homomorphisms between the underlying groups. Hence, there is an isomorphism between the category of groups and the category of discrete groups. Discrete groups can therefore be identified with their underlying (non-topological) groups.

There are some occasions when a topological group or Lie group is usefully endowed with the discrete topology, 'against nature'. This happens for example in the theory of the Bohr compactification, and in group cohomology theory of Lie groups.

A discrete isometry group is an isometry group such that for every point of the metric space the set of images of the point under the isometries is a discrete set. A discrete symmetry group is a symmetry group that is a discrete isometry group.

Properties

[edit]

Since topological groups are homogeneous, one need only look at a single point to determine if the topological group is discrete. In particular, a topological group is discrete only if the singleton containing the identity is an open set.

A discrete group is the same thing as a zero-dimensional Lie group (uncountable discrete groups are not second-countable, so authors who require Lie groups to have this property do not regard these groups as Lie groups). The identity component of a discrete group is just the trivial subgroup while the group of components is isomorphic to the group itself.

Since the only Hausdorff topology on a finite set is the discrete one, a finite Hausdorff topological group must necessarily be discrete. It follows that every finite subgroup of a Hausdorff group is discrete.

A discrete subgroup H of G is cocompact if there is a compact subset K of G such that HK = G.

Discrete normal subgroups play an important role in the theory of covering groups and locally isomorphic groups. A discrete normal subgroup of a connected group G necessarily lies in the center of G and is therefore abelian.

Other properties:

  • every discrete group is totally disconnected
  • every subgroup of a discrete group is discrete.
  • every quotient of a discrete group is discrete.
  • the product of a finite number of discrete groups is discrete.
  • a discrete group is compact if and only if it is finite.
  • every discrete group is locally compact.
  • every discrete subgroup of a Hausdorff group is closed.
  • every discrete subgroup of a compact Hausdorff group is finite.

Examples

[edit]
  • Frieze groups and wallpaper groups are discrete subgroups of the isometry group of the Euclidean plane. Wallpaper groups are cocompact, but Frieze groups are not.
  • A crystallographic group usually means a cocompact, discrete subgroup of the isometries of some Euclidean space. Sometimes, however, a crystallographic group can be a cocompact discrete subgroup of a nilpotent or solvable Lie group.
  • Every triangle group T is a discrete subgroup of the isometry group of the sphere (when T is finite), the Euclidean plane (when T has a Z + Z subgroup of finite index), or the hyperbolic plane.
  • Fuchsian groups are, by definition, discrete subgroups of the isometry group of the hyperbolic plane.
    • A Fuchsian group that preserves orientation and acts on the upper half-plane model of the hyperbolic plane is a discrete subgroup of the Lie group PSL(2,R), the group of orientation preserving isometries of the upper half-plane model of the hyperbolic plane.
    • A Fuchsian group is sometimes considered as a special case of a Kleinian group, by embedding the hyperbolic plane isometrically into three-dimensional hyperbolic space and extending the group action on the plane to the whole space.
    • The modular group PSL(2,Z) is thought of as a discrete subgroup of PSL(2,R). The modular group is a lattice in PSL(2,R), but it is not cocompact.
  • Kleinian groups are, by definition, discrete subgroups of the isometry group of hyperbolic 3-space. These include quasi-Fuchsian groups.
    • A Kleinian group that preserves orientation and acts on the upper half space model of hyperbolic 3-space is a discrete subgroup of the Lie group PSL(2,C), the group of orientation preserving isometries of the upper half-space model of hyperbolic 3-space.
  • A lattice in a Lie group is a discrete subgroup such that the Haar measure of the quotient space is finite.

See also

[edit]

Citations

[edit]
  1. ^ Pontrjagin 1946, p. 54.

References

[edit]
[edit]
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Discrete group

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In mathematics, a discrete group is a topological group endowed with the discrete topology.[1] Discrete groups frequently arise as discrete subgroups of larger Lie groups, providing a bridge between algebraic and geometric structures; for instance, the special linear group SL2(Z)\mathrm{SL}_2(\mathbb{Z}) serves as a classic example, acting discontinuously on the hyperbolic plane via Möbius transformations.[1] In geometric group theory, discrete groups are typically understood as countable groups with this topology, enabling the study of their large-scale geometry through tools like word metrics and Cayley graphs, which model the group's combinatorial structure.[2] These graphs, with vertices representing group elements and edges corresponding to generators, reveal quasi-isometric invariants that classify groups up to coarse equivalence, a cornerstone for analyzing fundamental groups of manifolds and solving problems in low-dimensional topology.[2] The significance of discrete groups extends to their role in symmetries of discrete objects, such as lattices, tilings, and polyhedra, where they generate fundamental domains and orbifolds in quotient spaces.[1] Finitely generated and finitely presented discrete groups, defined by finite sets of generators and relations, are particularly tractable, allowing computations of asphericity via associated 2-complexes.[2] Overall, discrete groups underpin advancements in areas like hyperbolic geometry and 3-manifold theory, influencing pure mathematics.[2]

Definition

Topological definition

A topological group $ G $ is called discrete if it is equipped with the discrete topology, in which every subset of $ G $ is open.[3][4] Equivalently, $ G $ is discrete if the identity element $ e $ has a neighborhood containing no other points of $ G $, meaning $ e $ is isolated and, by the homogeneity of topological groups, every point is isolated.[3] In the discrete topology on any set, the group operations of multiplication and inversion are automatically continuous, as all functions between discrete spaces are continuous.[5] A subgroup $ H $ of a topological group $ G $ is discrete if $ H $, endowed with the subspace topology induced from $ G $, has the discrete topology, so that every singleton $ { h } $ for $ h \in H $ is open in $ H $.[6] This means each point of $ H $ is isolated in the relative topology, ensuring no accumulation points within $ H $.[7] Unlike general topological groups, where the topology may allow points to accumulate and connected components to form, discreteness enforces complete separation of points while preserving the continuity of the group operations.[8]

Equivalent conditions

A topological group GG equipped with the discrete topology is discrete if and only if the identity element ee is an isolated point in GG, meaning there exists an open neighborhood UU of ee such that U∩G={e}U \cap G = \{e\}.[9] Since left translations by elements of GG are homeomorphisms, this condition at the identity extends to every point in GG being isolated, confirming that the subspace topology on GG is the full discrete topology.[9] For a subgroup HH of a topological group GG, an equivalent characterization of discreteness is that every sequence in HH consisting of distinct elements has no convergent subsequence in GG.[9] This follows from the fact that in the subspace topology on HH, discreteness implies no limit points within HH, and thus no non-constant convergent sequences in HH, with convergence in GG restricted to HH yielding the same property.[9] Such sequences diverge in GG, ensuring that HH inherits the discrete topology without accumulation points from the ambient space. In the special case where the topological group GG is metrizable, a subgroup HH is discrete if and only if there exists ε>0\varepsilon > 0 such that for every h∈Hh \in H, the open ball of radius ε\varepsilon centered at hh contains no other elements of HH.[9] This uniform separation condition leverages the metric to verify isolation of points in HH, equivalent to the topological definition via the first-countability of metric spaces, where sequences suffice to detect limit points.[9] These characterizations typically assume that the topological group GG is Hausdorff, as non-Hausdorff topologies can lead to pathologies where discrete subgroups fail to be closed or where isolation fails to propagate uniformly.[9] Standard treatments of topological groups, including those involving discrete subgroups, therefore impose the Hausdorff separation axiom to ensure well-behaved convergence and quotient structures.[9]

Properties

Topological properties

A discrete group, equipped with the discrete topology, is totally disconnected as a topological space, meaning that its only connected subsets are singletons; this follows from the fact that every singleton is both open and closed in the discrete topology.[5] Consequently, the connected component of the identity element consists solely of the identity itself.[10] Every discrete group is locally compact, since each point admits a compact neighborhood—namely, the singleton set containing that point, which is compact in the discrete topology.[10] Moreover, compact neighborhoods in a discrete group are precisely the finite subsets, as any infinite subset lacks a finite subcover for its open cover by singletons.[11] Finite discrete groups are therefore compact, while infinite ones are not.[5] Discrete groups are always Hausdorff, as the discrete topology separates distinct points with disjoint open singletons.[5] Their topology is zero-dimensional, possessing a basis of clopen sets (the singletons).[10] In particular, any countable discrete group is second-countable, rendering it a zero-dimensional Lie group.[12] Discrete groups admit a Bohr compactification, which is a compact Hausdorff group $ bG $ together with a continuous homomorphism $ \sigma: G \to bG $ that is injective on $ G $ and dense in $ bG $, such that every continuous homomorphism from $ G $ to a compact group factors through $ bG $.[13] For discrete $ G $, the image $ \sigma(G) $ is dense in the compact group $ bG $, extending the discrete topology continuously to this compactification.[14]

Algebraic properties

A discrete topological group inherits its algebraic structure from the underlying group operation, with the discrete topology ensuring that certain closure properties hold under compatible topologies. Specifically, any subgroup of a discrete topological group is itself discrete in the subspace topology, as the discrete topology on the ambient group restricts to the discrete topology on the subgroup.[7] Similarly, if NN is a closed normal subgroup of a discrete topological group GG, the quotient group G/NG/N equipped with the quotient topology is also discrete, since the cosets are open sets in GG and the quotient map is open.[7] In connected topological groups, normal discrete subgroups exhibit particularly restrictive algebraic behavior. A discrete normal subgroup NN of a connected topological group GG must be central, meaning that every element of NN commutes with every element of GG. This follows from the fact that conjugation maps g↦gng−1g \mapsto gng^{-1} for n∈Nn \in N are continuous homomorphisms from the connected group GG to the discrete group NN, hence constant, implying gng−1=ngng^{-1} = n for all g∈Gg \in G. Consequently, such subgroups are abelian, as central subgroups of any group are abelian. The discreteness of a group also imposes constraints on its cardinality when viewed as a subgroup of a larger space with countability axioms. In particular, every discrete subgroup of a second-countable locally compact group is countable.[15] This arises because the second-countability of the ambient group allows a countable basis, and the discreteness ensures that the subgroup injects into a countable collection of cosets via a fundamental domain of finite measure.[15] Regarding compactness, finite discrete groups are compact, as they are finite spaces in the discrete topology. However, infinite discrete groups cannot be compact in the Hausdorff sense, since the open cover by singletons admits no finite subcover.[7] This finiteness condition highlights the interplay between the algebraic infinitude of the group and its topological compactness, preventing infinite discrete groups from being compact without altering the topology.[7]

Examples

Discrete subgroups of the real line

The additive group of real numbers (R,+)(\mathbb{R}, +) endowed with its standard metric topology provides a simple setting to illustrate discrete subgroups. In this context, a subgroup H⊆RH \subseteq \mathbb{R} is discrete if it inherits the discrete topology from the subspace topology, meaning every point in HH is isolated; equivalently, for every h∈Hh \in H, there exists ϵ>0\epsilon > 0 such that the open ball B(h,ϵ)∩H={h}B(h, \epsilon) \cap H = \{h\}. This isolation condition holds precisely when the elements of HH are separated by a positive minimal distance.[16] A fundamental classification theorem states that every proper subgroup of (R,+)(\mathbb{R}, +) is either dense in R\mathbb{R} or cyclic, generated by a single nonnegative real number a≥0a \geq 0. The dense subgroups fail to be discrete, as their elements accumulate everywhere without isolation. In contrast, the cyclic subgroups aZ={ka∣k∈Z}a\mathbb{Z} = \{ka \mid k \in \mathbb{Z}\} (with a>0a > 0) are discrete, since consecutive elements are separated by distance aa, allowing an open ball of radius a/2a/2 around each point to contain no other subgroup elements. The case a=1a = 1 yields the integers Z\mathbb{Z} as a prototypical example, where the minimal distance between distinct points is 1, ensuring isolation in the subspace topology induced from R\mathbb{R}.[17] The trivial subgroup {0}\{0\} is also discrete, as its sole element is isolated by any positive-radius ball centered at 0 that intersects the subgroup only at itself. More generally, for any integer n>0n > 0, the subgroup nZn\mathbb{Z} is discrete with spacing nn, generated by nn and forming a lattice-like structure along the line. A key non-example is the rationals Q\mathbb{Q}, which is dense in R\mathbb{R} because between any two reals there lies a rational, violating the isolation condition and rendering Q\mathbb{Q} non-discrete.[17] This distance-based verification aligns with equivalent conditions for discreteness in metric topological groups like R\mathbb{R}, where closedness and proper separation suffice to confirm the discrete subspace topology.[16]

Discrete symmetry groups

Discrete symmetry groups arise as discrete subgroups of the isometry groups of Euclidean spaces, acting properly discontinuously to produce tilings with periodic symmetries. These groups capture the rigid motions—translations, rotations, reflections, and glide reflections—that preserve infinite patterns in the plane or space, fundamental to understanding geometric repetition and order. Frieze groups consist of discrete isometries of the Euclidean plane that preserve an infinite horizontal strip, such as a band of repeating motifs along a line. They are generated by translations along the strip and additional symmetries like rotations or reflections perpendicular to it, with the translation subgroup isomorphic to the integers Z\mathbb{Z}. There are precisely seven frieze groups, classified up to conjugacy by their geometric symmetries despite some algebraic isomorphisms. These include: (1) translations only (hop); (2) glide reflections (step); (3) translations and vertical reflections (sidle); (4) translations and 180° rotations (spinning hop); (5) glide reflections and 180° rotations (spinning sidle); (6) translations and horizontal reflections (jump); and (7) translations, horizontal and vertical reflections (all symmetries). Examples feature translations for basic repetition and glide reflections combining translation with reflection over a parallel line, enabling patterns like those in ancient Greek key designs or modern borders.[18] Extending to full plane symmetries, wallpaper groups are the 17 discrete subgroups of the Euclidean plane's isometry group that act periodically via a lattice of translations, enabling infinite tilings without gaps or overlaps. Classified by the International Union of Crystallography, they combine five lattice types—oblique, rectangular, centered rectangular, square, and hexagonal—with point group symmetries restricted by the crystallographic limitation theorem, allowing rotations only of orders 1, 2, 3, 4, or 6. Key elements include translations forming a rank-2 abelian subgroup, rotations about lattice points, reflections over lines, and glide reflections; notation like p4mm encodes primitive cell type, maximal rotation order, and mirror presence. These groups underpin the symmetry of two-dimensional periodic structures, from Islamic geometric art to molecular layers, where each wallpaper group dictates compatible tile shapes and orientations.[19] In three dimensions, crystallographic groups, or space groups, generalize this to discrete subgroups of the Euclidean space isometry group, generating periodic lattices for crystal structures. There are 230 such groups when distinguishing enantiomorphic pairs (mirror images), or 219 affine types otherwise, enumerated independently in the 1890s by W. Barlow, E.S. Federov, and A. Schönflies using extensions of two-dimensional classifications. They incorporate 14 Bravais lattices with 32 point groups, augmented by screw rotations and glide planes, finite in number due to the crystallographic restriction. These groups describe the symmetries of atomic arrangements in solids, emphasizing translational periodicity in all directions for tiling three-dimensional space.[20] A notable example beyond Euclidean space is the modular group PSL(2,Z)\mathrm{PSL}(2, \mathbb{Z}), the projective special linear group over the integers, which forms a discrete subgroup of PSL(2,R)\mathrm{PSL}(2, \mathbb{R}). It acts on the hyperbolic plane H2\mathbb{H}^2 via Möbius transformations, producing a fundamental domain like the standard modular region bounded by geodesics. This action yields a non-Euclidean tiling with triangular faces, highlighting discrete symmetries in curved geometries.[21] In contrast, the full rotation group SO(3)\mathrm{SO}(3) of three-dimensional Euclidean space is a connected Lie group, homeomorphic to the real projective space RP3\mathbb{RP}^3, and thus not discrete, as its continuous paths connect any two rotations without isolated elements.[22]

Other notable examples

Fuchsian groups are discrete subgroups of the projective special linear group PSL(2,R)\mathrm{PSL}(2, \mathbb{R}), which consists of orientation-preserving isometries of the hyperbolic plane H2\mathbb{H}^2.[23] These groups, introduced by Henri Poincaré in his 1882 memoir Théorie des groupes fuchsiens, play a key role in the uniformization of Riemann surfaces and the study of hyperbolic geometry.[24] A prominent subclass consists of Schottky groups, which are freely generated by hyperbolic elements pairing disjoint simple closed curves (circles) on the Riemann sphere, ensuring the group acts freely and properly discontinuously on H2\mathbb{H}^2.[25] Kleinian groups extend this concept to three dimensions, defined as discrete subgroups of PSL(2,C)\mathrm{PSL}(2, \mathbb{C}), the group of orientation-preserving isometries of hyperbolic 3-space H3\mathbb{H}^3.[26] Also originating from Poincaré's work in 1883, these groups are fundamental to the geometrization of 3-manifolds, where quotients H3/Γ\mathbb{H}^3 / \Gamma for a Kleinian group Γ\Gamma yield hyperbolic structures on manifolds.[27] Their limit sets on the sphere at infinity determine the topology and geometry of the quotients, with applications in Thurston's hyperbolization theorem. Free groups on a finite number of generators, when endowed with the discrete topology, form basic examples of discrete groups, as every singleton is open and the group action is properly discontinuous by definition.[28] These groups arise naturally in combinatorial group theory and as fundamental groups of surfaces with punctures. Profinite groups, constructed as inverse limits of finite groups each equipped with the discrete topology, yield discrete topological groups precisely when the resulting group is finite, such as the cyclic group of order nn as lim←⁡Z/pkZ\varprojlim \mathbb{Z}/p^k\mathbb{Z} for prime pp dividing nn.[29] Infinite profinite groups, like the pp-adic integers Zp\mathbb{Z}_p, carry a non-discrete compact topology. In contrast, not all subgroups of topological groups are discrete; for instance, the subgroup of the circle group T=R/Z\mathbb{T} = \mathbb{R}/\mathbb{Z} generated by an irrational rotation rα(x)=x+αmod  1r_\alpha(x) = x + \alpha \mod 1 with α\alpha irrational is dense in T\mathbb{T}, hence non-discrete.[30]

Discrete subgroups of Lie groups

Lattices and fundamental domains

In the context of Lie groups, a lattice is defined as a discrete subgroup Γ\Gamma of a locally compact Lie group GG such that the quotient space G/ΓG/\Gamma has finite Haar measure.[31] This finite-volume condition ensures that Γ\Gamma is "maximal" in a measure-theoretic sense among discrete subgroups, as the action of Γ\Gamma on GG partitions the space into regions of bounded total volume.[32] Lattices play a central role in the study of homogeneous spaces, where the quotient G/ΓG/\Gamma often forms a manifold with rich geometric structure. Lattices are classified as uniform or non-uniform based on the compactness of the quotient. A uniform lattice Γ\Gamma in GG yields a compact quotient G/ΓG/\Gamma, meaning the fundamental domain can be chosen to be compact, which implies Γ\Gamma acts cocompactly on GG.[33] In contrast, a non-uniform lattice produces a non-compact quotient with finite volume, often featuring cusps or ends where the group action thins out. A classic example is the modular group SL(2,Z)\mathrm{SL}(2, \mathbb{Z}) acting as a non-uniform lattice in SL(2,R)\mathrm{SL}(2, \mathbb{R}), where the quotient is the modular surface with a cusp at infinity.[31] Associated with any lattice Γ\Gamma in a Lie group GG is a fundamental domain D⊂GD \subset G, an open set such that the Γ\Gamma-translates γD\gamma D for γ∈Γ\gamma \in \Gamma tile GG without interior overlap and cover GG up to a set of measure zero. The domain DD has finite volume, matching that of G/ΓG/\Gamma, and serves as a "fundamental region" for reduction theory, allowing explicit descriptions of orbits and invariants under the group action. For rank-one semisimple Lie groups, such domains can be constructed explicitly using the geometry of symmetric spaces, facilitating the study of tiling properties and volume computations.[34] The Selberg trace formula provides a powerful tool for analyzing lattices in groups like PSL(2,R)\mathrm{PSL}(2, \mathbb{R}), which act on hyperbolic manifolds. For a lattice Γ\Gamma yielding a hyperbolic manifold M=H2/ΓM = \mathbb{H}^2 / \Gamma, the formula equates the trace of the heat kernel (related to Laplacian eigenvalues on MM) to a sum over lengths of closed geodesics in the fundamental domain, weighted by orbital contributions.[35] This spectral-geometric duality reveals how the distribution of eigenvalues encodes the geometry of the fundamental domain, with applications to eigenvalue estimates and prime geodesic theorems on such manifolds.[36] Rigidity theorems further highlight the structural stability of lattices in higher-rank semisimple Lie groups. The Mostow-Prasad rigidity theorem states that for irreducible lattices Γ1\Gamma_1 and Γ2\Gamma_2 in semisimple Lie groups G1G_1 and G2G_2 of rank at least two, with finite-volume quotients, any isomorphism Γ1≅Γ2\Gamma_1 \cong \Gamma_2 extends to a unique (up to conjugation) isomorphism G1≅G2G_1 \cong G_2. Originally proved by Mostow for cocompact lattices in 1968 and extended by Prasad to finite-volume cases in 1973, this result implies that the geometry and representation of such lattices are rigidly determined by their abstract group structure.

Arithmetic and congruence subgroups

Arithmetic groups are discrete subgroups of Lie groups that arise from algebraic groups defined over the rational numbers Q\mathbb{Q}. Specifically, a subgroup Γ\Gamma of a semisimple Lie group GG is arithmetic if it is commensurable with G(Z)G(\mathbb{Z}), where GG is an algebraic group defined over Q\mathbb{Q} embedded into G(R)G(\mathbb{R}), and commensurable means that the intersection of Γ\Gamma and G(Z)G(\mathbb{Z}) has finite index in both.[37] These groups are lattices, meaning they are discrete subgroups with finite covolume in GG.[37] A canonical example is SL(n,Z)\mathrm{SL}(n, \mathbb{Z}) as a discrete subgroup of SL(n,R)\mathrm{SL}(n, \mathbb{R}) for n≥2n \geq 2.[37] Congruence subgroups form an important class of finite-index subgroups within arithmetic groups. A congruence subgroup of an arithmetic group Γ=G(Z)\Gamma = G(\mathbb{Z}) is defined by congruence conditions modulo some integer mm, specifically as the preimage under the reduction map G(Z)→G(Z/mZ)G(\mathbb{Z}) \to G(\mathbb{Z}/m\mathbb{Z}).[37] The principal congruence subgroup Γ(m)\Gamma(m) is the kernel of this map, consisting of elements that reduce to the identity modulo mm.[37] For instance, in SL(2,Z)\mathrm{SL}(2, \mathbb{Z}), Γ(m)={A∈SL(2,Z)∣A≡I(modm)}\Gamma(m) = \{ A \in \mathrm{SL}(2, \mathbb{Z}) \mid A \equiv I \pmod{m} \}, which is normal and of index growing with mm.[37] Notable examples of arithmetic groups include the Bianchi groups, which are SL(2,Od)\mathrm{SL}(2, \mathcal{O}_d) where Od\mathcal{O}_d is the ring of integers in the imaginary quadratic field Q(−d)\mathbb{Q}(\sqrt{-d}) for square-free positive integers dd, acting discretely on hyperbolic 3-space.[38] Another class is the Hilbert modular groups, such as SL(2,OK)\mathrm{SL}(2, \mathcal{O}_K) for a totally real number field KK of degree greater than 1, where OK\mathcal{O}_K is the ring of integers of KK, embedding into products of SL(2,R)\mathrm{SL}(2, \mathbb{R}).[37] Margulis superrigidity provides a profound characterization: for irreducible lattices Γ\Gamma in a higher-rank semisimple Lie group GG over Q\mathbb{Q} (with real rank at least 2 and not isogenous to SO(1,n)×K\mathrm{SO}(1,n) \times K or SU(1,n)×K\mathrm{SU}(1,n) \times K), Γ\Gamma must be arithmetic.[37] This theorem implies that such lattices exhibit strong rigidity in their representations and homomorphisms.[37] The congruence subgroup problem investigates whether every finite-index subgroup of an arithmetic group Γ=G(k)\Gamma = G(k) (with kk a number field) contains a principal congruence subgroup of finite index, or equivalently, if all finite-index subgroups are congruence subgroups.[39] For SL(n,Z)\mathrm{SL}(n, \mathbb{Z}) with n≥3n \geq 3, the problem has a positive solution: every finite-index subgroup is a congruence subgroup.[39] In general, for semisimple groups of Q\mathbb{Q}-rank at least 2, the problem is affirmatively resolved, though counterexamples exist in lower ranks, such as non-congruence subgroups in SL(2,Z)\mathrm{SL}(2, \mathbb{Z}).[39]

Applications

In geometry and crystallography

In geometry, discrete groups play a fundamental role in describing the symmetries of periodic structures, particularly in crystallography where they classify the possible arrangements of atoms in crystals. The symmetry groups of crystals in three dimensions are known as space groups, which are discrete subgroups of the Euclidean motion group E(3)\mathbb{E}(3) that act properly discontinuously on R3\mathbb{R}^3. There are exactly 230 such space groups, encompassing all combinations of lattice types, point group symmetries, screw axes, and glide planes that preserve the crystal lattice.[40][41] Discrete groups also arise in the construction of manifolds and orbifolds through quotient spaces. When a discrete group Γ\Gamma acts freely and properly discontinuously on a manifold MM (i.e., Γ\Gamma is torsion-free), the quotient M/ΓM/\Gamma is a manifold without singularities. If Γ\Gamma has torsion elements, the quotient M/ΓM/\Gamma forms an orbifold, which generalizes manifolds by allowing isolated singular points corresponding to finite stabilizers.[42] In hyperbolic geometry, discrete groups such as Fuchsian and Kleinian groups generate important classes of surfaces and three-manifolds. A Fuchsian group is a discrete subgroup of PSL(2,R)\mathrm{PSL}(2,\mathbb{R}), acting on the hyperbolic plane H2\mathbb{H}^2, and its quotient by a fundamental domain yields a hyperbolic surface of finite area. Similarly, Kleinian groups, discrete subgroups of PSL(2,C)\mathrm{PSL}(2,\mathbb{C}) acting on hyperbolic three-space H3\mathbb{H}^3, produce hyperbolic three-manifolds via fundamental domains, providing models for low-dimensional topology.[43][44] The Bieberbach theorems characterize discrete groups acting on Euclidean space. These theorems state that for a discrete subgroup Γ\Gamma of the Euclidean group E(n)\mathbb{E}(n) acting properly discontinuously and cocompactly on Rn\mathbb{R}^n, the translation subgroup Γtrans\Gamma_\mathrm{trans} is a lattice isomorphic to Zn\mathbb{Z}^n of full rank, and Γ\Gamma is a semidirect product Zn⋊F\mathbb{Z}^n \rtimes F where FF is a finite point group acting faithfully on the lattice. Such groups, known as Euclidean crystallographic groups, underpin the symmetry of periodic tilings in nn-dimensions.[45][46] Beyond periodic crystals, discrete groups model aperiodic structures in physical materials like quasicrystals, which exhibit long-range order without translational periodicity. Quasicrystals are described by diffraction patterns arising from tilings generated by discrete symmetry operations, often involving icosahedral or five-fold rotational symmetries incompatible with lattice periodicity. A prominent example is the Penrose tiling of the plane, constructed via inflation rules that substitute larger prototiles with clusters of smaller ones, yielding aperiodic tilings with discrete rotational symmetries and modeling the atomic arrangements in real quasicrystalline alloys.[47][48][49]

In number theory and arithmetic geometry

In number theory, discrete groups play a central role in the study of modular forms, which are automorphic forms defined on quotients of the upper half-plane H\mathbb{H} by congruence subgroups Γ\Gamma of SL(2,Z)\mathrm{SL}(2, \mathbb{Z}).[50] These forms, such as Eisenstein series, exhibit transformation properties under the action of Γ\Gamma, enabling the construction of spaces of cusp forms whose dimensions are governed by arithmetic invariants like the level and weight.[50] Arithmetic subgroups like SL(2,Z)\mathrm{SL}(2, \mathbb{Z}) provide the foundational discrete structure for these quotients, linking holomorphic properties to number-theoretic phenomena such as the distribution of primes.[51] Discrete groups also connect to L-functions and zeta functions through cohomology of arithmetic groups like GL(n,Z)\mathrm{GL}(n, \mathbb{Z}). The residue of the Dedekind zeta function of a number field at s=1s=1 involves the class number, which is interpreted via the idele class group in class field theory and reflects the structure of ideals.[52] For higher-rank groups, automorphic L-functions attached to representations of GL(n,AQ)/GL(n,Q)\mathrm{GL}(n, \mathbb{A}_\mathbb{Q})/\mathrm{GL}(n, \mathbb{Q}) encode analytic continuations and functional equations, where GL(n,Q)\mathrm{GL}(n, \mathbb{Q}) embeds discretely and relates to cohomology classes that compute special values.[53] In arithmetic geometry, étale fundamental groups serve as profinite discrete groups that classify Galois representations associated to varieties over number fields. The étale fundamental group π1eˊt(XK‾)\pi_1^{\text{ét}}(X_{\overline{K}}) of a scheme XX over a number field KK is profinite and captures unramified covers, with continuous representations ρ:π1eˊt(XK‾)→GLn(Q‾ℓ)\rho: \pi_1^{\text{ét}}(X_{\overline{K}}) \to \mathrm{GL}_n(\overline{\mathbb{Q}}_\ell) corresponding to l-adic cohomology sheaves.[54] These groups, introduced by Grothendieck, replace topological fundamental groups and enable the study of arithmetic invariants like the conductor through their profinite topology.[55] The Langlands program establishes deep links between representations of arithmetic discrete groups and automorphic forms on adelic quotients. It conjectures correspondences between irreducible Galois representations of Gal(Q‾/Q)\mathrm{Gal}(\overline{\mathbb{Q}}/\mathbb{Q}) (a profinite group) and cuspidal automorphic representations of GL(n,AQ)/GL(n,Q)\mathrm{GL}(n, \mathbb{A}_\mathbb{Q})/\mathrm{GL}(n, \mathbb{Q}), where GL(n,Q)\mathrm{GL}(n, \mathbb{Q}) embeds discretely.[56] These correspondences preserve L-functions and imply reciprocity laws, unifying number theory with representation theory via the adelic framework.[57] The Shafarevich theorem, proved by Faltings, asserts the finiteness of isomorphism classes of abelian varieties over a number field with bounded conductor, relying on discrete group actions in Galois cohomology. Specifically, it bounds the set of such varieties up to isomorphism by controlling torsors under the action of the absolute Galois group, whose profinite structure ensures only finitely many isomorphism classes satisfy the conductor condition.[58] This finiteness extends to semiabelian varieties and underpins results on the arithmetic of Jacobians.[58]

References

  1. Discrete Group -- from Wolfram MathWorld
  2. On the Geometry and Topology of Discrete Groups: An Overview
  3. [PDF] A Crash Course in Topological Groups - Cornell Mathematics
  4. None
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