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In mathematics, the equivariant algebraic K-theory is an algebraic K-theory associated to the category of equivariant coherent sheaves on an algebraic scheme X with action of a linear algebraic group G, via Quillen's Q-construction; thus, by definition,
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In particular, is the Grothendieck group of . The theory was developed by R. W. Thomason in 1980s.[1] Specifically, he proved equivariant analogs of fundamental theorems such as the localization theorem.
Equivalently, may be defined as the of the category of coherent sheaves on the quotient stack .[2][3] (Hence, the equivariant K-theory is a specific case of the K-theory of a stack.)
A version of the Lefschetz fixed-point theorem holds in the setting of equivariant (algebraic) K-theory.[4]
Let X be an equivariant algebraic scheme.
Localization theorem — Given a closed immersion of equivariant algebraic schemes and an open immersion , there is a long exact sequence of groups
One of the fundamental examples of equivariant K-theory groups are the equivariant K-groups of -equivariant coherent sheaves on a points, so . Since is equivalent to the category of finite-dimensional representations of . Then, the Grothendieck group of , denoted is .[5]
Given an algebraic torus a finite-dimensional representation is given by a direct sum of -dimensional -modules called the weights of .[6] There is an explicit isomorphism between and given by sending to its associated character.[7]