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Irreducible representation

In representation theory, an irreducible representation of a group G is a homomorphism \rho: G \to \mathrm{GL}(V) from G to the general linear group of invertible linear transformations on a vector space V (typically over the complex numbers \mathbb{C}) such that the only subspaces of V invariant under the action of \rho(G) are the trivial subspace \{0\} and V itself.^[1]^[2] This property distinguishes irreducible representations as the "indecomposable" or fundamental units that cannot be broken down into simpler, nontrivial subrepresentations.^[3]^[4] Irreducible representations play a pivotal role in understanding group symmetries across mathematics and physics, serving as the basic building blocks for all finite-dimensional representations of finite groups.^[4] By Maschke's theorem, over a field whose characteristic does not divide the order of G, every finite-dimensional representation decomposes uniquely (up to isomorphism and ordering) as a direct sum of irreducible representations.^[2]^[3] For finite groups, the number of distinct irreducible representations (up to isomorphism) equals the number of conjugacy classes in G, and the sum of the squares of their dimensions equals the order of G.^[1]^[2] These representations are analyzed through their characters, which are the traces of the matrices \rho(g) for g \in G and provide orthogonality relations that facilitate decomposition and classification.^[2] Examples include the standard n-dimensional representation of the orthogonal group O(n) on \mathbb{R}^n, which is irreducible, and the one-dimensional trivial representation where every group element acts as the identity.^[1] In applications, such as quantum mechanics and particle physics, irreducible representations classify symmetry operations and predict physical states invariant under group actions.^[4] For Lie groups and their algebras, similar decomposition principles hold under appropriate conditions, extending the theory to continuous symmetries.^[1]

Fundamentals

Historical Development

The concept of irreducible representations emerged in the late 19th century as part of the foundational work in group theory, particularly through the efforts of Ferdinand Georg Frobenius. In 1896, Frobenius introduced the notion of group characters in his seminal paper "Über Gruppencharaktere," where he analyzed the group determinant—a polynomial associated with a finite group—and demonstrated that it factors into irreducible factors corresponding to the group's irreducible representations.^[5] This work extended earlier ideas from abelian groups and laid the groundwork for classifying irreducible representations of finite groups by showing that the number of such representations equals the number of conjugacy classes.^[5] Over the following years, from 1896 to 1903, Frobenius further developed character theory, establishing orthogonality relations and proving that characters of irreducible representations form an orthonormal basis for class functions, which enabled the complete decomposition of any representation into irreducibles.^[6] Shortly thereafter, in 1898, Heinrich Maschke proved that, under appropriate conditions on the field, every finite-dimensional representation of a finite group decomposes uniquely (up to isomorphism) as a direct sum of irreducible representations—a result now known as Maschke's theorem. Building on Frobenius's foundations and Maschke's theorem, Issai Schur advanced the theory in the early 20th century by shifting focus from permutation representations to abstract linear representations over the complex numbers. In his 1904 paper "Über die Darstellung der endlichen Gruppen durch gebrochene lineare Substitutionen," Schur provided criteria for irreducibility, including Schur's lemma, which states that any endomorphism of an irreducible representation is a scalar multiple of the identity.^[7] He extended this in subsequent works, such as the 1905 and 1907 papers "Untersuchungen über die Darstellung der endlichen Gruppen durch gebrochene lineare Substitutionen," where he utilized Maschke's theorem and developed methods to construct irreducible representations explicitly using Young's tableaux for symmetric groups.^[8] By 1911, Schur's contributions had solidified the framework for finite groups, emphasizing the role of characters in determining irreducibility and multiplicity.^[9] In the 1920s, Hermann Weyl extended these ideas from finite and permutation groups to continuous groups, particularly compact Lie groups and Lie algebras, marking a transition to modern representation theory. Weyl's four groundbreaking papers from 1925 to 1926 introduced the Weyl character formula, which computes the character of any irreducible representation in terms of its highest weight, integrating root systems and the Weyl group.^[10] He also developed the "unitarian trick" to prove complete reducibility for semisimple Lie groups by averaging over the compact form and collaborated with Fritz Peter on the 1926 Peter-Weyl theorem, establishing that irreducible characters form an orthonormal basis for L² functions on compact groups.^[10] These advancements bridged discrete and continuous settings, influencing applications in quantum mechanics and symmetric spaces.^[11]

Notation and Terminology for Group Representations

In representation theory, a representation of a finite group G on a vector space V over a field F is defined as a group homomorphism \rho: G \to \mathrm{GL}(V), where \mathrm{GL}(V) denotes the general linear group consisting of all invertible linear endomorphisms of V.^[12] This homomorphism assigns to each element g \in G an invertible linear transformation \rho(g) \in \mathrm{GL}(V) such that \rho(gh) = \rho(g) \rho(h) for all g, h \in G, and \rho(e) = I_V where e is the identity element and I_V is the identity transformation on V. Equivalently, a representation provides a linear action of G on V, meaning each group element acts via a linear transformation preserving the vector space structure.^[13] Upon choosing a basis for V, the representation \rho yields a matrix representation, which is a homomorphism \rho: G \to \mathrm{GL}(n, F) where n = \dim V, with each \rho(g) expressed as an invertible n \times n matrix over F.^[12] Standard notation includes \rho(g) for the image of g \in G, and the dimension of the representation, denoted \dim(\rho) or simply n, equals \dim V.^[13] If V is equipped with an inner product (e.g., a Hermitian inner product over \mathbb{C}), the representation is called unitary if each \rho(g) preserves the inner product, meaning \rho(g) is a unitary operator for every g \in G.^[12] Two representations \rho: G \to \mathrm{GL}(V) and \sigma: G \to \mathrm{GL}(W) are equivalent if there exists an invertible linear map T: V \to W (a similarity transformation) such that \sigma(g) = T \rho(g) T^{-1} for all g \in G.^[14] More generally, a linear map T: V \to W is an intertwining operator between \rho and \sigma if it commutes with the group action, i.e., T \rho(g) = \sigma(g) T for all g \in G; equivalence holds if such a T is invertible.^[15] While representations of groups are a special case, they can be reformulated as modules over the group algebra F[G], where the action of group elements extends linearly to the algebra, though the focus here remains on the homomorphic view for groups.^[13]

Definitions

Reducible and Irreducible Representations

In the context of a representation \rho: [G](/page/G) \to \mathrm{[GL](/page/GL)}(V) of a group [G](/page/G) on a finite-dimensional vector space V over a field k, a subspace W \subseteq V is invariant if \rho(g)W \subseteq W for all g \in [G](/page/G).^[16] The representation \rho is reducible if it admits a proper nontrivial invariant subspace W, meaning $0 \subsetneq W \subsetneq V; otherwise, \rho is irreducible.^[16] Equivalently, \rho is reducible if there exists a proper subrepresentation, that is, the restriction \rho|_W: [G](/page/G) \to \mathrm{[GL](/page/GL)}(W) for some proper invariant W \neq 0, V.^[16] Under conditions ensuring complete reducibility, such as Maschke's theorem for finite groups over fields where the characteristic does not divide the group order, if \rho is reducible with invariant subspace W, then there exists a complementary invariant subspace U \subseteq V such that

V = W \oplus U,

where both W and U are invariant under \rho, and the restrictions \rho|_W and \rho|_U are subrepresentations of \rho. This decomposition allows the representation to be "reduced" by studying the smaller subrepresentations separately, motivating the focus on irreducible representations as the fundamental building blocks in representation theory. The invariant subspace criterion is central to identifying reducibility, and reduction can often be achieved explicitly using projection operators. Suppose W is a given invariant subspace and P_0: V \to W is any projection onto W (satisfying P_0^2 = P_0 and \mathrm{im}\, P_0 = W). For a finite group G with |G| invertible in k (i.e., \mathrm{char}\, k \nmid |G|), define the averaged operator

P = \frac{1}{|G|} \sum_{g \in G} \rho(g) \circ P_0 \circ \rho(g)^{-1}.

This P is a projection onto W that intertwines the action of G, meaning \rho(h) \circ P = P \circ \rho(h) for all h \in G, so \ker P provides an invariant complement to W. Such projections enable the block-diagonalization of the representing matrices in a basis respecting the decomposition V = W \oplus \ker P. Maschke's theorem establishes that, under the same field characteristic condition, every finite-dimensional representation of a finite group is completely reducible: it decomposes as a direct sum of irreducible representations. This result, proved using averaging techniques similar to the projection construction above, underscores the importance of irreducibility by guaranteeing that all representations can be built from irreducibles without "entangled" components.

Decomposable and Indecomposable Representations

In representation theory, a representation \rho: G \to \mathrm{GL}(V) of a group G over a field k, equivalently a module over the group algebra kG, is called decomposable if V is isomorphic to a direct sum W \oplus U of two nonzero kG-submodules, meaning both W and U are invariant under the action of G.^[17] Conversely, the representation is indecomposable if it cannot be expressed as such a direct sum of two nonzero subrepresentations.^[17] This notion arises naturally in the study of modules over algebras, where every finite-dimensional module decomposes uniquely (up to isomorphism and ordering) into a direct sum of indecomposable modules by the Krull-Schmidt theorem, provided the endomorphism rings of the indecomposables are local rings.^[17] A key characterization is that a finite-dimensional representation V is indecomposable if and only if its endomorphism ring \mathrm{End}_{kG}(V) is a local ring, meaning it has a unique maximal ideal (the non-units form an ideal).^[18] The concepts of decomposability and indecomposability differ from reducibility and irreducibility, as the latter concern the existence of a single proper invariant subspace rather than a complemented direct sum decomposition. Indecomposability is a weaker condition than irreducibility: every irreducible representation is indecomposable (since it has no proper subrepresentations at all), but the converse fails in non-semisimple settings.^[17] For instance, over an algebraically closed field of characteristic zero, finite-dimensional representations of finite groups are completely reducible by Maschke's theorem, so every representation decomposes into a direct sum of irreducibles, and thus indecomposability coincides with irreducibility.^[17] However, in characteristic p > 0 dividing the group order, or for representations of algebras without semisimple module categories, indecomposable representations that are reducible (i.e., possessing proper invariant subspaces without invariant complements) abound.^[17] A classic example occurs in modular representation theory for p-groups. Consider the cyclic group G = \mathbb{Z}/p\mathbb{Z} over the field k = \mathbb{F}_p of characteristic p, with the two-dimensional representation V = k^2 where the generator g acts via the matrix

\begin{pmatrix} 1 & 1 \\ 0 & 1 \end{pmatrix}.

The subspace W = \operatorname{span}\{(1,0)\} is invariant under this action, since g \cdot (1,0)^T = (1,0)^T, making V reducible. However, there is no invariant complement to W in V, as any potential complement would not be preserved by g; thus, V is indecomposable.^[19] This Jordan block-like structure exemplifies how, in non-semisimple categories such as p-restricted representations of finite-dimensional algebras (including group algebras in characteristic p), direct sum decompositions may fail even when invariant subspaces exist.^[17] Such examples highlight the role of indecomposables in classifying representations beyond the semisimple case.

Connection Between Irreducibility and Indecomposability

In representation theory, every irreducible representation of a group G on a vector space V is indecomposable. This follows because irreducibility means V has no proper nonzero G-invariant subspaces, so it cannot decompose as a direct sum V = U \oplus W where both U and W are nonzero G-invariant subspaces; such a decomposition would contradict the absence of proper subrepresentations.^[6] Indecomposability, however, is a weaker condition: a representation is indecomposable if it is not isomorphic to a direct sum of two nonzero subrepresentations, but it may still admit proper subrepresentations that do not have invariant complements.^[6] The converse—that every indecomposable representation is irreducible—does not hold in general but is true under semisimple conditions on the representation category. For instance, for finite groups over the complex numbers \mathbb{C}, Maschke's theorem implies that the group algebra \mathbb{C}[G] is semisimple, so every finite-dimensional representation is completely reducible into a direct sum of irreducibles; thus, indecomposables coincide with irreducibles.^[6] Similarly, this holds for representations of finite groups over fields of characteristic not dividing |G|.^[20] Counterexamples arise in modular representation theory, where the characteristic p of the field divides |G|, leading to nonsemisimple categories with indecomposable but reducible representations. A classic example is the two-dimensional permutation representation of S_3 over \mathbb{F}_2: it has a one-dimensional trivial subrepresentation but cannot be expressed as a direct sum of two nonzero subrepresentations, making it indecomposable yet reducible.^[20] Such modules are common for p-groups, where only the trivial representation is irreducible, but higher-dimensional uniserial modules are indecomposable with nontrivial composition series.^[20] The Krull-Schmidt theorem provides a canonical framework for decomposition in these settings: for artinian modules (such as finite-dimensional representations over a field), any direct sum decomposition into indecomposable summands is unique up to isomorphism and reordering of factors.^[6] This uniqueness aids in classifying representations, even when indecomposables exceed irreducibles.^[20] A key structural consequence of irreducibility is that the endomorphism algebra \operatorname{End}_G(V) forms a division algebra, meaning every nonzero endomorphism has an inverse; this reflects the absence of nontrivial invariant subspaces and underpins many orthogonality relations.^[6]

Examples

Trivial and One-Dimensional Representations

The trivial representation of a finite group G is the one-dimensional representation \rho: G \to \mathrm{GL}(1, \mathbb{C}) given by \rho(g) = [1](/page/1) for all g \in G. This representation acts on the complex vector space \mathbb{C} by sending every group element to multiplication by the identity scalar 1, and it is always irreducible because the underlying space has dimension 1, admitting no proper nontrivial invariant subspaces.^[21] More generally, any one-dimensional representation of a finite group G over \mathbb{C} is a group homomorphism \rho: G \to \mathbb{C}^\times, where \mathbb{C}^\times denotes the multiplicative group of nonzero complex numbers. Such representations are matrix-valued functions taking values in $1 \times 1 matrices, and they are irreducible by virtue of their dimension: the only subspaces of a one-dimensional vector space are the zero subspace and the full space itself, both of which are trivially invariant.^[22] For finite abelian groups, all irreducible representations over \mathbb{C} are one-dimensional; this follows from the commutativity of the group, which implies that every irreducible representation must be scalar (by Schur's lemma applied to commuting operators) and thus one-dimensional. In this context, the irreducible representations coincide with the characters of the group, forming the dual group \hat{G} under pointwise multiplication, and there are exactly |G| such representations. For a one-dimensional representation \rho, the character function satisfies \chi(g) = \rho(g) for all g \in G, as the trace of the $1 \times 1 matrix \rho(g) is simply the scalar itself.^[21] A concrete example is the sign representation of the symmetric group S_3, which sends even permutations to 1 and odd permutations (transpositions) to -1; this is a nontrivial one-dimensional irreducible representation distinct from the trivial one. For the cyclic group C_n = \langle r \rangle of order n, the irreducible representations are the n one-dimensional characters \rho_k(r^m) = \exp(2\pi i k m / n) for k = 0, 1, \dots, n-1, where \rho_0 is the trivial representation and the others provide the full set of roots of unity characters.^[22]^[23]

Irreducible Representations over the Complex Numbers

In the complex numbers, an algebraically closed field of characteristic zero, every finite-dimensional representation of a finite group G is completely reducible into a direct sum of irreducible representations.^[24] For such groups, the irreducible representations over \mathbb{C} are classified by character theory: the number of distinct irreducible representations (up to isomorphism) equals the number of conjugacy classes in G, and the dimension of each irreducible representation divides |G|.^[24] A key formula arising from the orthogonality of characters states that for the character \chi of an irreducible representation, \sum_{g \in G} |\chi(g)|^2 = |G|.^[24] This relation underscores how character values determine the representation's dimension \chi(1) and confirm irreducibility. A concrete non-abelian example is the symmetric group S_3 of order 6, which has three conjugacy classes (the identity, the class of transpositions, and the class of 3-cycles) and thus three irreducible representations over \mathbb{C}: the one-dimensional trivial representation, the one-dimensional sign representation, and the two-dimensional standard representation.^[25] The dimensions 1, 1, and 2 all divide 6, consistent with the general classification. The standard representation acts on the two-dimensional subspace \{(x,y,z) \in \mathbb{C}^3 \mid x + y + z = 0\} of the natural permutation representation, and it is faithful. Explicit matrices in this representation, with respect to a suitable basis, are as follows for the generators (123) and (12):

\rho((123)) = \begin{pmatrix} -\frac{1}{2} & -\frac{\sqrt{3}}{2} \\ \frac{\sqrt{3}}{2} & -\frac{1}{2} \end{pmatrix}, \quad \rho((12)) = \begin{pmatrix} 0 & 1 \\ 1 & 0 \end{pmatrix}.

These matrices extend by group relations to the full representation, which is irreducible over \mathbb{C}.^[25] Another illustrative non-abelian example is the quaternion group Q_8 = \{\pm 1, \pm i, \pm j, \pm k\} of order 8, which has five conjugacy classes ({1}, {-1}, {\pm i}, {\pm j}, {\pm k}) and thus five irreducible representations over \mathbb{C}: four one-dimensional representations and one two-dimensional representation.^[26] The dimensions 1, 1, 1, 1, and 2 all divide 8. The one-dimensional representations factor through the abelianization Q_8 / \{\pm 1\} \cong \mathbb{Z}_2 \times \mathbb{Z}_2, providing the trivial representation and three nontrivial ones where elements outside the kernels act by -1. The two-dimensional representation is faithful and arises from the inclusion of Q_8 in the unit quaternions; with respect to the basis {1, j} of the quaternions as a complex vector space, the matrices are:

\rho(i) = \begin{pmatrix} i & 0 \\ 0 & -i \end{pmatrix}, \quad \rho(j) = \begin{pmatrix} 0 & -1 \\ 1 & 0 \end{pmatrix}, \quad \rho(k) = \begin{pmatrix} 0 & -i \\ -i & 0 \end{pmatrix},

with \rho(-x) = -\rho(x) for x = i, j, k and \rho(\pm 1) = \pm I_2. This representation is irreducible over \mathbb{C}.^[26]

Irreducible Representations over Finite Fields

Irreducible representations of finite groups over finite fields of characteristic p, where p is prime, form the subject of modular representation theory, which addresses cases where p divides the group order and classical theorems like Maschke's semisimple decomposition fail. In this context, the group algebra kG (with k algebraically closed of characteristic p) may not be semisimple, leading to indecomposable but reducible modules. Brauer theory organizes these representations into p-blocks, which are the indecomposable two-sided ideals of kG annihilating the simple modules, partitioning the irreducible representations according to linked composition factors in projectives.^[27]^[28] A key example illustrates the differences from characteristic zero: for the symmetric group S_3 over a field of characteristic 3 (such as \mathbb{F}_3), there are two irreducible representations, both one-dimensional—the trivial representation and the sign representation—corresponding to the two p-regular conjugacy classes (the identity and the class of transpositions). The two-dimensional irreducible representation from the complex case collapses into the direct sum of these two one-dimensional representations, as the action of 3-cycles becomes unipotent and the module decomposes.^[27]^[29] For cyclic groups C_n over \mathbb{F}_p, the irreducible representations remain one-dimensional when p \nmid n, matching the characteristic zero count of n distinct characters. However, when p \mid n, such as for C_p, there is only one irreducible representation, the trivial one, since all non-identity elements are p-singular. In this case, projective indecomposables play a central role; the unique projective indecomposable module is the group algebra itself (regular module), which has composition length p with p-1 trivial factors in its socle and head.^[30]^[27] The number of irreducible modular representations equals the number of p-regular conjugacy classes, which may differ from the number of ordinary irreducible representations over \mathbb{C}. Ordinary characters reduce modulo p via a lift to a p-adic ring, but these reductions are typically not irreducible and decompose into modular irreducibles. The decomposition matrix D = (d_{ij}) encodes this relation, where each ordinary irreducible character \chi_i expresses as \chi_i = \sum_j d_{ij} \phi_j on p-regular elements, with \phi_j the Brauer characters of modular irreducibles and d_{ij} \geq 0 integers (usually 0 or 1, but not always). For S_3 in characteristic 3, labeling rows by the ordinary characters (trivial \chi_1, sign \chi_2, standard \chi_3) and columns by modular Brauer characters \phi_1 (trivial), \phi_2 (sign),

D = \begin{pmatrix} 1 & 0 \\ 0 & 1 \\ 1 & 1 \end{pmatrix},

reflecting the standard character's decomposition.^[27]^[28]

Properties

Schur's Lemma

Schur's lemma is a fundamental result in representation theory that characterizes the linear maps, known as intertwiners, between irreducible representations of a group. Consider a group G acting via representations \rho: G \to \mathrm{GL}(V) and \sigma: G \to \mathrm{GL}(W) on finite-dimensional vector spaces V and W over a field k. An intertwiner is a linear map T: V \to W such that T \rho(g) = \sigma(g) T for all g \in G. If \rho and \sigma are irreducible but not equivalent, then the space of intertwiners \mathrm{Hom}_G(V, W) = \{0\}. If \rho and \sigma are equivalent irreducible representations, then \mathrm{Hom}_G(V, W) is isomorphic to a division algebra over k; in particular, over an algebraically closed field such as \mathbb{C}, this space is one-dimensional, consisting of scalar multiples of isomorphisms.^[31]^[12] A key special case arises when V = W and \rho = \sigma, so T is an endomorphism commuting with the representation. For an irreducible representation \rho over an algebraically closed field k, any such T satisfying [T, \rho(g)] = 0 for all g \in G must be a scalar multiple of the identity: T = \lambda I for some \lambda \in k. This follows from the general version, as the endomorphism ring \mathrm{End}_G(V) is then a division algebra over an algebraically closed field, hence one-dimensional and consisting of scalars.^[31]^[12] To sketch the proof, suppose T: V \to W is a nonzero intertwiner between irreducible representations over any field. The kernel \ker T and image \mathrm{im} T are G-invariant subspaces of V and W, respectively. By irreducibility, \ker T = \{0\} (so T is injective) and \mathrm{im} T = W (so T is surjective), hence T is an isomorphism. For the endomorphism case over an algebraically closed field, if T is not a scalar, consider T - \lambda I for some eigenvalue \lambda of T; its kernel is a proper nonzero invariant subspace, contradicting irreducibility. Thus, all eigenvalues coincide, and T = \lambda I.^[31]^[12] An important application concerns unitary representations over \mathbb{C}. For a finite-dimensional irreducible unitary representation \pi: G \to U(V) of a group G, any two \pi-invariant Hermitian inner products on V differ by a scalar multiple. Indeed, if \langle \cdot, \cdot \rangle_1 and \langle \cdot, \cdot \rangle_2 are such inner products, the associated sesquilinear form map T: V \to V defined by \langle Tv, w \rangle_1 = \langle v, w \rangle_2 is an intertwiner (positive definite by unitarity), hence T = \lambda I with \lambda > 0. This establishes the uniqueness of the unitary form up to scaling.^[32]

Orthogonality Relations for Characters

The character of a representation \rho: G \to \mathrm{GL}(V) of a finite group G over \mathbb{C} is defined as the function \chi_\rho: G \to \mathbb{C} given by \chi_\rho(g) = \mathrm{tr}(\rho(g)) for each g \in G, where \mathrm{tr} denotes the trace.^[20] This function is a class function, meaning it is constant on conjugacy classes of G, and it determines the representation up to isomorphism when \rho is irreducible. The space of class functions on G is equipped with the inner product

\langle \chi, \psi \rangle = \frac{1}{|G|} \sum_{g \in G} \chi(g) \overline{\psi(g)},

where \overline{\psi(g)} is the complex conjugate.^[20] This inner product induces a Hermitian structure, and for characters of irreducible representations, it yields key orthogonality properties. The first orthogonality relation states that if \rho and \sigma are irreducible representations of G, then

\frac{1}{|G|} \sum_{g \in G} \chi_\rho(g) \overline{\chi_\sigma(g)} = \delta_{\rho \sigma},

where \delta_{\rho \sigma} = 1 if \rho \cong \sigma and $0 otherwise. This shows that the characters of distinct irreducibles are orthogonal with respect to the inner product. The second orthogonality relation concerns sums over irreducible characters: for g, h \in G,

\sum_{\rho} \chi_\rho(g) \overline{\chi_\rho(h)} = \frac{|G|}{|C_G(g)|} \delta_{\mathrm{cl}(g), \mathrm{cl}(h)},

where the sum runs over all irreducible representations \rho (up to isomorphism), C_G(g) is the centralizer of g in G, and \delta_{\mathrm{cl}(g), \mathrm{cl}(h)} = 1 if g and h are conjugate and $0 otherwise.^[20] These relations can be proved using Schur's lemma applied to the regular representation of G. The regular representation decomposes as a direct sum of each irreducible representation \rho with multiplicity equal to its dimension \dim \rho = \chi_\rho(1). Consider the operator on this space induced by left multiplication by a fixed element; Schur's lemma implies that intertwiners between distinct irreducibles vanish, leading to the orthogonality of traces (characters) when summing over the group.^[20] For the second relation, conjugation by group elements preserves the decomposition, and centralizer sizes account for the action within classes. As consequences, the irreducible characters form an orthonormal basis for the space of class functions on G with respect to the inner product, and they constitute a complete set, spanning all such functions. This basis property enables the decomposition of any representation via its character and facilitates computations like the number of irreducibles equaling the number of conjugacy classes.^[20]

Complete Reducibility for Compact Groups

In the theory of group representations, compact groups possess a fundamental property known as complete reducibility, which states that every finite-dimensional continuous representation over the complex numbers is a direct sum of irreducible representations.^[33] This result, often attributed to Hermann Weyl, extends the discrete case of Maschke's theorem for finite groups to the continuous setting and relies crucially on the topological structure of compact groups.^[34] The proof begins with the construction of a G-invariant inner product on the representation space V. Given any inner product ⟨⋅,⋅⟩₀ on V, define a new inner product by averaging over the group using the normalized Haar measure μ (with μ(G)=1):

\langle v, w \rangle = \int_G \langle \rho(g)v, \rho(g)w \rangle_0 \, d\mu(g),

where ρ: G → GL(V) is the representation. This integral converges due to compactness, and the resulting inner product is Hermitian positive definite and invariant under ρ(g) for all g ∈ G, making the representation unitary.^[33] With this invariant inner product, any invariant subspace U ⊆ V has an orthogonal complement U^⊥ that is also invariant, allowing the iterative decomposition of V into a direct sum of irreducible subrepresentations.^[34] To explicitly decompose V, one projects onto its isotypic components (multiplicities of each irreducible ρ). The orthogonal projection operator onto the ρ-isotypic component is given by

P_\rho = (\dim \rho) \int_G \overline{\chi_\rho(g)} \, \rho(g) \, d\mu(g),

where χ_ρ is the character of ρ; this generalizes the finite-group formula and is G-equivariant by Schur's lemma.^[35] These projections are mutually orthogonal and sum to the identity on V, yielding the complete decomposition V ≅ ⨁_ρ (V_ρ ⊗ Hom_G(V_ρ, V)), where the sum is finite.^[33] This property connects to the Peter–Weyl theorem, which asserts that the Hilbert space L²(G) decomposes as a direct sum ⊕_ρ (V_ρ ⊗ V_ρ^*) over all irreducible representations ρ, with matrix coefficients forming an orthonormal basis; this infinite-dimensional analog underscores the complete reducibility in the finite-dimensional case.^[34] In contrast, non-compact groups like SL(2,ℝ) admit finite-dimensional representations that are indecomposable but not completely reducible, such as certain extensions of the trivial representation.^[33]

Applications

In Physics and Chemistry

In quantum mechanics, irreducible representations (irreps) of symmetry groups provide a fundamental framework for classifying quantum states according to their transformation properties under group operations. For instance, the irreps of the rotation group SO(3) correspond directly to the angular momentum quantum number l = 0, 1, 2, \dots, where each irrep has dimension $2l + 1 and labels the degeneracy of energy levels in central potential problems, such as the hydrogen atom.^[36] This classification arises from the complete reducibility of representations for compact groups like SO(3), ensuring that physical observables, such as the Hamiltonian, commute with symmetry operations and thus preserve irrep labels.^[36] A key application is in deriving selection rules for transition probabilities, where matrix elements \langle \psi | O | \phi \rangle between states \psi and \phi vanish unless the irreps of the states are contained in the decomposition of the direct product involving the irrep of the operator O, as determined by Clebsch-Gordan coefficients.^[37] This principle, encapsulated in the Wigner-Eckart theorem, enforces symmetries in processes like atomic transitions and scattering, preventing forbidden interactions. The foundational role of group theory in quantum mechanics was established by Eugene Wigner in his 1931 monograph, which systematically applied irreps to atomic spectra and symmetry principles. In chemistry, irreps of molecular point groups classify vibrational modes, enabling the analysis of symmetry-adapted normal coordinates for polyatomic molecules. For water (H₂O), which belongs to the C_{2v} point group, the three vibrational modes transform as the irreducible representations A₁ (symmetric stretch), A₁ (bending), and B₂ (asymmetric stretch), all one-dimensional due to the group's Abelian nature.^[38] Character tables of these point groups further determine spectroscopic activity: a mode is infrared (IR) active if it matches the symmetry of the dipole moment (e.g., A₁, B₁, B₂ in C_{2v} for H₂O, allowing all modes to be IR active), while Raman activity requires matching the polarizability tensor symmetries (e.g., A₁ and B₂ for H₂O).^[39] This symmetry-based selection ensures that only specific modes contribute to observed spectra, aiding in molecular identification and structural elucidation.^[39]

Irreducible Representations of Lie Groups

Finite-dimensional irreducible representations of compact Lie groups are classified by dominant integral weights within the root systems associated to their Lie algebras. For a semisimple compact Lie group G with Lie algebra \mathfrak{g}, the root system \Delta decomposes the dual space \mathfrak{h}^* of the Cartan subalgebra \mathfrak{h}, and the positive roots \Delta^+ define a Weyl chamber. A weight \lambda \in \mathfrak{h}^* is dominant if it lies in the closure of the fundamental Weyl chamber, satisfying \langle \lambda, \alpha \rangle \geq 0 for all positive roots \alpha \in \Delta^+, where \langle \cdot, \cdot \rangle denotes the pairing induced by the Killing form. Each such dominant integral weight \lambda parametrizes a unique finite-dimensional irreducible representation L(\lambda), up to isomorphism, which is highest weight with highest weight vector annihilated by the positive root spaces.^[40] A concrete example arises for the special unitary group \mathrm{SU}(2), whose Lie algebra \mathfrak{su}(2) has root system of type A_1. The irreducible representations are labeled by non-negative half-integers j = 0, 1/2, 1, 3/2, \dots, with dimension $2j + 1. These correspond to symmetric powers of the defining representation on \mathbb{C}^2, where the highest weight is $2j times the fundamental weight, and the representation space decomposes into weight spaces with weights -2j, -2j+2, \dots, 2j. This classification extends the general highest weight theory, illustrating how root multiplicities and Weyl group orbits determine the full weight structure.^[40] Infinitesimal representations of Lie groups are studied through representations of their Lie algebras, where a representation \rho: \mathfrak{g} \to \mathfrak{gl}(V) captures the local action near the identity. For semisimple \mathfrak{g}, Casimir operators—central elements in the universal enveloping algebra U(\mathfrak{g}), such as the quadratic Casimir C = \sum_i x_i x^i with respect to an invariant bilinear form—act as scalars on irreducible representations, providing invariants like the eigenvalue \langle \lambda + 2\rho, \lambda \rangle for highest weight \lambda, where \rho is half the sum of positive roots. These operators distinguish representations and facilitate computations of dimensions via Weyl's character formula.^[40]^[41] Weyl's unitary trick enables the construction of finite-dimensional irreducible representations for non-compact semisimple Lie groups by complexifying the Lie algebra and embedding into a compact real form, where representations are unitary and thus completely reducible. For instance, the complexification of \mathfrak{sl}(n, \mathbb{R}) yields \mathfrak{sl}(n, \mathbb{C}), whose representations restrict to those of the compact \mathrm{SU}(n). This method, originally developed by Hermann Weyl, leverages the averaging operator over the compact group with respect to the Haar measure to project onto invariant subspaces, ensuring irreducibility in the highest weight modules upon restriction.^[40]^[42]

The Lorentz Group

The Lorentz group, denoted SO(3,1), is the group of spacetime symmetries preserving the Minkowski metric in special relativity, and its irreducible representations play a central role in classifying fields and particles in relativistic quantum field theory. The finite-dimensional irreducible representations of SO(3,1) are labeled by pairs of non-negative half-integers or integers (m, n), where the dimension of the representation is (2m+1)(2n+1). These representations are non-unitary due to the non-compact nature of the group but are crucial for describing tensor fields and spinors. They arise from the isomorphism of the complexified Lorentz algebra so(3,1)_{\mathbb{C}} with sl(2,\mathbb{C}) \oplus sl(2,\mathbb{C}), where the (m, n) representation corresponds to the tensor product of the spin-m and spin-n representations of the two SL(2,\mathbb{C}) factors.^[43] The universal double cover of the proper orthochronous Lorentz group SO^+(3,1) is SL(2,\mathbb{C}), which facilitates the classification: the (m, n) representations transform under SL(2,\mathbb{C}) \times SL(2,\mathbb{C}) as the bifundamental (2m+1) \otimes (2n+1), with m and n determining the eigenvalues of the Casimir operators associated with each sl(2,\mathbb{C}) factor, given by m(m+1) and n(n+1). The Lie algebra generators consist of rotation generators J_i and boost generators K_i (for i=1,2,3), satisfying the commutation relations

[J_i, J_j] = i \epsilon_{ijk} J_k, \quad [K_i, K_j] = -i \epsilon_{ijk} J_k, \quad [J_i, K_j] = i \epsilon_{ijk} K_k,

which reflect the boost-rotation mixing characteristic of the Lorentz algebra. A representative example is the vector representation, corresponding to (1/2, 1/2), which acts on 4-component Minkowski vectors and has dimension 4. Another key example is the Dirac spinor representation, obtained as the direct sum (1/2, 0) \oplus (0, 1/2), also 4-dimensional, describing 4-component spinor fields that combine left- and right-handed Weyl spinors.^[43]^[44] For physical applications requiring unitarity, such as quantum fields, the relevant irreducible representations are infinite-dimensional unitary ones, classified by Eugene Wigner using the method of induced representations from the little group of the Poincaré group, though the Lorentz subgroup structure yields the principal series. These unitary representations form continuous series parameterized by a complex parameter \nu with \operatorname{Re} \nu = 1/2 (principal series) or within complementary series bounds, acting on Hilbert spaces of functions on the hyperboloid or light cone, and are essential for massless fields with helicity. The principal series includes representations induced from characters of the maximal compact subgroup SO(3), ensuring irreducibility and unitarity for the non-compact boosts.^[45]^[46]

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