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Conjugacy class

In group theory, a conjugacy class of an element g in a group G is the set of all elements conjugate to g, consisting of elements of the form x^{-1}gx for x \in G. Conjugation defines an on the elements of G, partitioning the group into disjoint conjugacy classes where elements within the same class share similar algebraic properties, such as having the same . The size of a conjugacy class of g equals the index of the centralizer Z(g) = \{ x \in G \mid xg = gx \} in G, and thus divides the order of G. In abelian groups, every element forms its own singleton conjugacy class, while non-abelian groups exhibit larger classes that reflect the group's structure. The collection of conjugacy classes satisfies the class equation |G| = \sum |Cl(g_i)|, where the sum is over representatives g_i of each class, providing a fundamental tool for analyzing group orders and symmetries. Conjugacy classes play a central role in , where irreducible representations are constant on classes, and in the study of group actions, as they correspond to orbits under the conjugation action. For finite groups, the number and sizes of classes yield insights into solvability and other structural features, with explicit computations possible for small groups like the S_3, which has three classes: the , transpositions, and 3-cycles.

Fundamentals

Definition

In group theory, two elements g and h in a group G are said to be conjugate, written h \sim g, if there exists an k \in G such that h = k^{-1} g k. This operation defines the standard notion of right conjugation in groups. Left conjugation, by contrast, is given by h = k g k^{-1}, but the resulting sets of conjugate elements coincide because every group element admits an , so substituting k' = k^{-1} yields the same collection. The conjugacy class of an g \in G, commonly denoted \mathrm{Cl}(g) or , is the set of all elements conjugate to g, namely \mathrm{Cl}(g) = \{ k^{-1} g k \mid k \in G \}. Other notations, such as g^G for the class generated by conjugation from G, appear in some contexts. The conjugacy relation \sim is an on the set G. Reflexivity holds since g = e^{-1} g e for the e \in G. Symmetry follows because if h = k^{-1} g k, then g = k h k^{-1} = (k^{-1})^{-1} h (k^{-1}). Transitivity is verified by noting that if h = k^{-1} g k and m = l^{-1} h l, then m = (k l)^{-1} g (k l).

Examples

A concrete example of conjugacy classes arises in the symmetric group S_3, which consists of all permutations of three elements and has order 6. The conjugacy classes in S_3 are determined by cycle type: the identity element forms its own class \{e\} of size 1; the three transpositions (1\,2), (1\,3), and (2\,3) form a single class of size 3, as any transposition is conjugate to any other via an appropriate permutation; and the two 3-cycles (1\,2\,3) and (1\,3\,2) form another class of size 2. These classes partition S_3 and illustrate how elements of the same cycle structure are conjugate. In the quaternion group Q_8 = \{\pm 1, \pm i, \pm j, \pm k\} of order 8, the conjugacy classes are \{1\}, \{-1\}, \{i, -i\}, \{j, -j\}, and \{k, -k\}. The center Z(Q_8) = \{1, -1\} consists of elements that commute with everything, so $1 and -1 each form classes. The remaining elements pair with their negatives under conjugation: for instance, j \cdot i \cdot j^{-1} = -i, and similarly for the other pairs, yielding classes of size 2. These five classes reflect the non-abelian structure of Q_8. For any abelian group G, the conjugacy classes are all singletons \{g\} for each g \in G. This follows because, in an abelian group, elements commute, so x g x^{-1} = g for all x, g \in G, meaning no element is conjugate to a distinct one. Consequently, the number of conjugacy classes equals the order of G, and this singleton property characterizes abelian groups among all groups. In the general linear group \mathrm{GL}(n, F) over a field F, conjugacy classes correspond precisely to similarity classes of invertible n \times n matrices. Two matrices A, B \in \mathrm{GL}(n, F) are conjugate if there exists invertible P \in \mathrm{GL}(n, F) such that B = P^{-1} A P, which is the definition of similarity; this equivalence preserves properties like eigenvalues and Jordan form. For example, in \mathrm{GL}(2, \mathbb{C}), classes include diagonalizable matrices with distinct eigenvalues, scalar matrices, and non-diagonalizable Jordan blocks, each classified by their characteristic polynomials or minimal polynomials.

Core Properties

Basic Properties

In group theory, the conjugacy class of an element g in a group G, denoted \mathrm{Cl}(g), consists of all elements of the form k^{-1} g k for k \in G. These classes partition the group G into disjoint subsets, meaning every element of G belongs to exactly one conjugacy class, and the classes are pairwise disjoint. The centralizer of g in G, denoted Z_G(g) or C_G(g), is the subgroup defined by Z_G(g) = \{ k \in G \mid k^{-1} g k = g \}, which is equivalently the set of elements that commute with g. The size of the conjugacy class \mathrm{Cl}(g) is given by the index of the centralizer in G: |\mathrm{Cl}(g)| = [G : Z_G(g)] = |G| / |Z_G(g)|. This follows from the orbit-stabilizer theorem applied to the conjugation action, where Z_G(g) acts as the stabilizer of g. The center of the group, Z(G) = \{ z \in G \mid z k = k z \ \forall k \in G \}, is the union of all conjugacy classes, as each central z satisfies k^{-1} z k = z for all k \in G, making \mathrm{Cl}(z) = \{ z \}. Conjugacy classes are invariant under inner s of G, which are the automorphisms of the form \phi_k: x \mapsto k^{-1} x k for fixed k \in G; applying such an automorphism maps each class to itself.

Conjugacy Class Equation

The conjugacy class equation is a fundamental theorem in finite group theory that expresses the order of a finite group G as the sum of the orders of its distinct conjugacy classes. Specifically, if \{ g_1, g_2, \dots, g_k \} is a set of representatives, one from each conjugacy class of G, then |G| = \sum_{i=1}^k |\mathrm{Cl}(g_i)|, where \mathrm{Cl}(g_i) denotes the conjugacy class of g_i. This equation arises from applying the orbit-stabilizer theorem to the conjugation action of G on itself. For each element g \in G, the orbit under this action is precisely the conjugacy class \mathrm{Cl}(g), and the stabilizer is the centralizer C_G(g) of g in G. The orbit-stabilizer theorem thus yields |\mathrm{Cl}(g)| = |G| / |C_G(g)|. Summing this relation over one representative from each conjugacy class partitions G into disjoint classes and recovers the class equation. A key implication is that each conjugacy class size divides the of the group, since |C_G(g)| is the of the centralizer in G, and thus |\mathrm{Cl}(g)| divides |G|. This divisibility property provides constraints on the possible structures of s. The conjugacy class equation emerged in the 19th-century development of finite group theory, with foundational contributions from , who introduced the notion of conjugate permutations in 1844.

Equation Illustration

To illustrate the conjugacy class equation, consider the dihedral group D_4 of order 8, which consists of the symmetries of a square: four rotations and four reflections. This non-abelian group provides a concrete example where the equation |G| = \sum |Cl(g)| (sum over class representatives g) can be computed explicitly, revealing the structure through class sizes greater than 1. The elements of D_4 are denoted as e (identity), r (90° rotation), r^2 (180° rotation), r^3 (270° rotation), and reflections s, sr, sr^2, sr^3, satisfying r^4 = e, s^2 = e, and srs^{-1} = r^{-1}. To determine the conjugacy classes, compute the orbit of each element under conjugation, or equivalently, use the formula |Cl(g)| = |D_4| / |C_{D_4}(g)|, where C_{D_4}(g) is the centralizer of g. The identity e commutes with all elements, so C_{D_4}(e) = D_4 (order 8) and Cl(e) = \{e\} (size 1). The element r^2 lies in the center Z(D_4) = \{e, r^2\} and also commutes with everything, yielding C_{D_4}(r^2) = D_4 (order 8) and Cl(r^2) = \{r^2\} (size 1). For r, the centralizer is the rotation subgroup \langle r \rangle = \{e, r, r^2, r^3\} (order 4), since reflections conjugate r to r^3 (e.g., s r s^{-1} = r^{-1} = r^3), but rotations fix it. Thus, |Cl(r)| = 8/4 = 2, and Cl(r) = \{r, r^3\}. By symmetry, Cl(r^3) = Cl(r). The reflections split into two classes. For s (reflection over a horizontal axis), the centralizer is \{e, r^2, s, sr^2\} (order 4), as it includes the center and reflections over parallel axes; conjugations by rotations yield sr^2, while others map to the other type. Thus, |Cl(s)| = 8/4 = 2 and Cl(s) = \{s, sr^2\}. Similarly, for sr (diagonal reflection), C_{D_4}(sr) = \{e, r^2, sr, sr^3\} (order 4), giving Cl(sr) = \{sr, sr^3\} (size 2). The conjugacy classes are therefore \{e\}, \{r^2\}, \{r, r^3\}, \{s, sr^2\}, and \{sr, sr^3\}, with sizes 1, 1, 2, 2, 2. The class equation verifies: $8 = 1 + 1 + 2 + 2 + 2. The presence of three classes of size 2 highlights the non-abelian nature, as all classes would be singletons in an abelian group.

Group Action Perspective

Conjugacy as Group Action

The conjugation provides a natural framework for understanding conjugacy classes through the lens of group actions. Consider a group G acting on itself by conjugation, defined by the map \phi: G \times G \to G where \phi(k, g) = k^{-1} g k for k, g \in G. This satisfies the axioms: the acts as the identity map, and the action is compatible with the group operation in G. Under this action, the orbit of an element g \in G is the set \{ k^{-1} g k \mid k \in G \}, which is precisely the conjugacy class of g. Thus, the conjugacy classes of G partition G into the orbits of this conjugation action. The stabilizer of g under this action is the centralizer C_G(g) = \{ k \in G \mid k^{-1} g k = g \}, the subgroup of elements that commute with g. By the orbit-stabilizer theorem, the size of the conjugacy class of g is |G| / |C_G(g)|, assuming G is finite. This corresponds to the \psi: G \to \mathrm{Aut}(G) given by \psi(k)(g) = k^{-1} g k, whose image is the group \mathrm{Inn}(G) and is the center Z(G) = \{ z \in G \mid zg = gz \ \forall g \in G \}. Consequently, the conjugation factors through the of \mathrm{Inn}(G) on G.

Subgroups and Subsets

In group theory, the concept of conjugacy extends naturally from individual elements to subgroups. For a subgroup H of a group G and an element g \in G, the conjugate subgroup H^g is defined as H^g = g^{-1} H g = \{ g^{-1} h g \mid h \in H \}. This set H^g is itself a subgroup of G isomorphic to H, preserving the group structure under conjugation. The conjugacy classes of subgroups arise from the action of G by conjugation on the of its subgroups. Specifically, two subgroups H and K of G belong to the same conjugacy class if there exists g \in G such that K = H^g; the conjugacy class of H is then the \{ H^g \mid g \in G \} under this . A subgroup H forms a singleton conjugacy class—meaning it is fixed under conjugation by every element of G, so H^g = H for all g \in G—precisely when H is in G. This conjugation action applies more broadly to arbitrary subsets of G. For a subset S \subseteq G and g \in G, the conjugate S^g is S^g = g^{-1} S g = \{ g^{-1} s g \mid s \in S \}. The conjugacy classes of such subsets are the orbits under the conjugation , partitioning the power set of G into equivalence classes where subsets are related by relabeling via inner automorphisms. A significant application of subgroup conjugacy appears in the , which describe the structure of s via their Sylow p-subgroups for primes p dividing the group order. The second Sylow theorem states that all Sylow p-subgroups of a G are conjugate to each other, forming a single conjugacy class under the action of G by conjugation. This conjugacy ensures that Sylow p-subgroups are indistinguishable up to relabeling, providing a tool to classify group structures and count such subgroups modulo p.

Interpretations and Applications

Geometric Interpretation

In the general linear group \mathrm{GL}(n, \mathbb{R}), conjugacy classes correspond to similarity classes of matrices, where two matrices A and B are conjugate if there exists an P \in \mathrm{GL}(n, \mathbb{R}) such that B = P^{-1} A P. Geometrically, this equivalence means that A and B describe the same linear transformation relative to different bases, preserving the intrinsic structure of eigenspaces and generalized eigenspaces in the \mathbb{R}^n. Over algebraically closed fields such as \mathbb{C}, these classes are determined by the Jordan canonical form, which decomposes the matrix into a block-diagonal arrangement of Jordan blocks; each block visualizes a chain of generalized eigenvectors associated with an eigenvalue, highlighting the of the (geometric multiplicity) and the full eigenspace deficiency relative to diagonalizability. This form provides a spatial to the "shape" of the transformation, where larger blocks indicate longer chains of non-trivial actions beyond mere scaling. In the symmetric group S_n, conjugacy classes are indexed by partitions of n corresponding to cycle types, which geometrically represent the permutation as a disjoint union of cycles acting on the set \{1, 2, \dots, n\}, akin to orbiting subsets or loops in a graphical decomposition. Two permutations belong to the same class if they induce the same pattern of cycle lengths, interpretable as the topology of the permutation's action: for instance, a cycle of length k traces a k-sided polygon in the functional graph, while multiple cycles of equal length suggest symmetric rearrangements like parallel circuits. This cycle-type classification offers a visual pattern recognition, where the partition visually encodes the "skeleton" of the permutation's dynamics without regard to labeling of elements. A particularly intuitive visualization arises in S_3, isomorphic to the of symmetries of an , where the three conjugacy classes align with geometric transformation types: the (cycle type $1+1+1), the three transpositions (cycle type $2+1) as reflections over altitudes, and the two 3-cycles (cycle type $3) as 120° and 240° rotations around the . This separation geometrically distinguishes orientation-preserving motions (rotations, forming the alternating subgroup A_3) from orientation-reversing flips (reflections), illustrating how conjugacy classes capture distinct "motions" in the of a spatial object. For Lie groups, conjugacy classes in the group G correspond to adjoint orbits in the Lie algebra \mathfrak{g} under the adjoint action \mathrm{Ad}_g(X) = g X g^{-1} for X \in \mathfrak{g}, providing a differential geometric interpretation as submanifolds foliating \mathfrak{g}. These orbits geometrically represent the "level sets" of the adjoint representation, where each orbit is a homogeneous space diffeomorphic to G / Z_G(X) (with Z_G(X) the centralizer), and their symplectic structure (via Kirillov-Kostant-Souriau) endows them with a phase-space analogy, visualizing the decomposition of elements into semisimple and nilpotent parts through coadjoint orbits in \mathfrak{g}^*. In compact Lie groups, such orbits are compact Kähler manifolds, offering a spatial embedding that highlights the curvature and connectivity induced by the group's exponential map.

Representations Connection

In the representation theory of finite groups over the complex numbers, characters play a central role in connecting conjugacy classes to irreducible representations. A character \chi_V of a representation (V, \rho) of a finite group G is defined by \chi_V(g) = \operatorname{tr}(\rho(g)) for g \in G, and it is a class function, meaning \chi_V(hgh^{-1}) = \chi_V(g) for all h \in G. Thus, the value of \chi_V is constant on each conjugacy class of G, allowing characters to be viewed as functions on the set of conjugacy classes rather than individual elements. The space of all class functions, denoted \operatorname{cf}(G), forms a vector space of dimension equal to the number of conjugacy classes in G. The irreducible characters \{\chi_i\}, where each \chi_i is the character of an irreducible representation, form an orthonormal basis for \operatorname{cf}(G) with respect to the inner product \langle \phi, \psi \rangle = \frac{1}{|G|} \sum_{g \in G} \phi(g) \overline{\psi(g)}. Since class functions are constant on conjugacy classes, this inner product can be rewritten as a sum over conjugacy classes: \langle \chi_i, \chi_j \rangle = \frac{1}{|G|} \sum_{\text{classes } Cl} |\mathrm{Cl}| \chi_i(\mathrm{Cl}) \overline{\chi_j(\mathrm{Cl})}, where the orthogonality yields \langle \chi_i, \chi_j \rangle = \delta_{ij}. Equivalently, the unnormalized orthogonality relation is \sum_{\text{classes } Cl} \chi_i(\mathrm{Cl}) \overline{\chi_j(\mathrm{Cl})} |\mathrm{Cl}| = |G| \delta_{ij}. This basis property implies that the number of irreducible representations (up to isomorphism) equals the dimension of \operatorname{cf}(G), which is the number of conjugacy classes in G. The Frobenius-Schur indicator provides further insight into the nature of irreducible representations by detecting whether they are realizable over the real numbers, with a direct computation involving character values on conjugacy classes. For an irreducible character \chi of G, the indicator is \varepsilon(\chi) = \frac{1}{|G|} \sum_{g \in G} \chi(g^2), which takes values in \{0, 1, -1\}. This sum groups terms by conjugacy classes of squares g^2, but more importantly, \varepsilon(\chi) = 1 if and only if \chi is real-valued (i.e., \chi(g) \in \mathbb{R} for all g \in G, hence on all conjugacy classes) and the representation is realizable over \mathbb{R} with a symmetric invariant bilinear form; \varepsilon(\chi) = -1 if \chi is real-valued but the representation requires quaternions with a skew-symmetric form; and \varepsilon(\chi) = 0 otherwise, indicating no real realization. Real conjugacy classes, those satisfying \mathrm{Cl}(g) = \mathrm{Cl}(g^{-1}), are precisely the classes on which all irreducible characters take real values, linking the indicator to the group's real class structure. A key application arises from these relations in counting irreducible representations via averaging over the group. The orthogonality of irreducible characters implies that the regular representation decomposes as a direct sum of each irreducible representation with multiplicity equal to its degree, leading to the identity |G| = \sum_i \chi_i(1)^2, where the sum is over irreducible characters. By projecting onto class functions using the inner product (an averaging process), one recovers the basis property and confirms that the number of irreducible representations equals the number of conjugacy classes, a fundamental result in character theory often proved by decomposing the space of class functions. This averaging technique underscores how conjugacy classes parametrize the irreducible representations, enabling explicit computations of character tables for groups like symmetric groups.