In group theory, restriction forms a representation of a subgroup using a known representation of the whole group. Restriction is a fundamental construction in representation theory of groups. Often the restricted representation is simpler to understand. Rules for decomposing the restriction of an irreducible representation into irreducible representations of the subgroup are called branching rules, and have important applications in physics. For example, in case of explicit symmetry breaking, the symmetry group of the problem is reduced from the whole group to one of its subgroups. In quantum mechanics, this reduction in symmetry appears as a splitting of degenerate energy levels into multiplets, as in the Stark or Zeeman effect. The induced representation is a related operation that forms a representation of the whole group from a representation of a subgroup. The relation between restriction and induction is described by Frobenius reciprocity and the Mackey theorem. Restriction to a normal subgroup behaves particularly well and is often called Clifford theory after the theorem of A. H. Clifford. Restriction can be generalized to other group homomorphisms and to other rings. For any group G, its subgroup H, and a linear representation ρ of G, the restriction of ρ to H, denoted is a representation of H on the same vector space by the same operators: Classical branching rules Classical branching rules describe the restriction of an irreducible complex representation (, V) of a classical group G to a classical subgroup H, i.e. the multiplicity with which an irreducible representation (σ, W) of H occurs in . By Frobenius reciprocity for compact groups, this is equivalent to finding the multiplicity of in the unitary representation induced from σ. Branching rules for the classical groups were determined by between successive unitary groups; between successive special orthogonal groups and unitary symplectic groups; from the unitary groups to the unitary symplectic groups and special orthogonal groups. The results are usually expressed graphically using Young diagrams to encode the signatures used classically to label irreducible representations, familiar from classical invariant theory. Hermann Weyl and Richard Brauer discovered a systematic method for determining the branching rule when the groups G and H share a common maximal torus: in this case the Weyl group of H is a subgroup of that of G, so that the rule can be deduced from the Weyl character formula. A systematic modern interpretation has been given by in the context of his