# Representation of a Lie group

In mathematics and theoretical physics, a **representation of a Lie group** is a linear action of a Lie group on a vector space. Equivalently, a representation is a smooth homomorphism of the group into the group of invertible operators on the vector space. Representations play an important role in the study of continuous symmetry. A great deal is known about such representations, a basic tool in their study being the use of the corresponding 'infinitesimal' representations of Lie algebras.

Each representation of a Lie group *G* gives rise to a representation of its Lie algebra; this correspondence is discussed in detail in subsequent sections. See representation of Lie algebras for the Lie algebra theory.

As noted above, the finite-dimensional representations of SO(3) arise naturally when studying the time-independent Schrödinger equation for a radial potential, such as the hydrogen atom, as a reflection of the rotational symmetry of the problem. (See the role played by the spherical harmonics in the mathematical analysis of hydrogen.)

In this section, we describe three basic operations on representations.^{[7]} See also the corresponding constructions for representations of a Lie algebra.

Certain types of Lie groups—notably, compact Lie groups—have the property that *every* finite-dimensional representation is isomorphic to a direct sum of irreducible representations.^{[2]} In such cases, the classification of representations reduces to the classification of irreducible representations. See Weyl's theorem on complete reducibility.

The tensor product of two irreducible representations is usually not irreducible; a basic problem in representation theory is then to decompose tensor products of irreducible representations as a direct sum of irreducible subspaces. This problem goes under the name of "addition of angular momentum" or "Clebsch–Gordan theory" in the physics literature.

In many cases, it is convenient to study representations of a Lie group by studying representations of the associated Lie algebra. In general, however, not every representation of the Lie algebra comes from a representation of the group. This fact is, for example, lying behind the distinction between integer spin and half-integer spin in quantum mechanics. On the other hand, if *G* is a simply connected group, then a theorem^{[11]} says that we do, in fact, get a one-to-one correspondence between the group and Lie algebra representations.

The Lie correspondence gives results only for the connected component of the groups, and thus the other components of the full group are treated separately by giving representatives for matrices representing these components, one for each component. These form (representatives of) the zeroth homotopy group of G. For example, in the case of the four-component Lorentz group, representatives of space inversion and time reversal must be put in by hand. Further illustrations will be drawn from the representation theory of the Lorentz group below.

A pictorial view of how the universal covering group contains *all* such homotopy classes, and a technical definition of it (as a set and as a group) is given in geometric view.

If *G* is a connected compact Lie group, its finite-dimensional representations can be decomposed as direct sums of irreducible representations.^{[17]} The irreducibles are classified by a "theorem of the highest weight." We give a brief description of this theory here; for more details, see the articles on and the parallel theory .

Two representations with the same character turn out to be isomorphic. Furthermore, the Weyl character formula gives a remarkable formula for the character of a representation in terms of its highest weight. Not only does this formula gives a lot of useful information about the representation, but it plays a crucial role in the proof of the theorem of the highest weight.

Here are some important examples in which unitary representations of a Lie group have been analyzed.

We have already discussed the irreducible projective unitary representations of the rotation group SO(3) above; considering projective representations allows for fractional spin in addition to integer spin.