Representation ring

In mathematics, especially in the area of algebra known as representation theory, the representation ring (or Green ring after J. A. Green) of a group is a ring formed from all the (isomorphism classes of the) finite-dimensional linear representations of the group. Elements of the representation ring are sometimes called virtual representations. For a given group, the ring will depend on the base field of the representations. The case of complex coefficients is the most developed, but the case of algebraically closed fields of characteristic "p" where the Sylow "p"-subgroups are cyclic is also theoretically approachable.
Formal definition.
Given a group "G" and a field "F", the elements of its representation ring "R""F"("G") are the formal differences of isomorphism classes of finite dimensional linear "F"-representations of "G". For the ring structure, addition is given by the direct sum of representations, and multiplication by their tensor product over "F". When "F" is omitted from the notation, as in "R"("G"), then "F" is implicitly taken to be the field of complex numbers.
Succinctly, the representation ring of "G" is the Grothendieck ring of the category of finite-dimensional representations of "G".
Characters.
Any representation defines a character χ:"G" → C. Such a function is constant on conjugacy classes of "G", a so-called class function; denote the ring of class functions by "C"("G"). If "G" is finite, the homomorphism "R"("G") → "C"("G") is injective, so that "R"("G") can be identified with a subring of "C"("G"). For fields "F" whose characteristic divides the order of the group "G", the homomorphism from "R""F"("G") → "C"("G") defined by Brauer characters is no longer injective.
For a compact connected group "R"("G") is isomorphic to the subring of "R"("T") (where "T" is a maximal torus) consisting of those class functions that are invariant under the action of the Weyl group (Atiyah and Hirzebruch, 1961). For the general compact Lie group, see Segal (1968).
λ-ring and Adams operations.
Given a representation of "G" and a natural number "n", we can form the "n"-th exterior power of the representation, which is again a representation of "G". This induces an operation λ"n" : "R"("G") → "R"("G"). With these operations, "R"("G") becomes a λ-ring.
The "Adams operations" on the representation ring "R"("G") are maps Ψ"k" characterised by their effect on characters χ:
The operations Ψ"k" are ring homomorphisms of "R"("G") to itself, and on representations ρ of dimension "d"
where the Λ"i"ρ are the exterior powers of ρ and "N""k" is the "k"-th power sum expressed as a function of the "d" elementary symmetric functions of "d" variables.