Equivariant sheaf

On the stalk level, the cocycle condition says that the isomorphism ${\displaystyle F_{gh\cdot x}\simeq F_{x}}$ is the same as the composition ${\displaystyle F_{g\cdot h\cdot x}\simeq F_{h\cdot x}\simeq F_{x}}$; i.e., the associativity of the group action. The unitarity of a group action is also a consequence: apply ${\displaystyle (e\times e\times 1)^{*},e:S\to G}$ to both sides to get ${\displaystyle (e\times 1)^{*}\phi \circ (e\times 1)^{*}\phi =(e\times 1)^{*}\phi }$ and so ${\displaystyle (e\times 1)^{*}\phi }$ is the identity.

Note that ${\displaystyle \phi }$ is an additional data; it is "a lift" of the action of G on X to the sheaf F. Moreover, when G is a connected algebraic group, F an invertible sheaf and X is reduced, the cocycle condition is automatic: any isomorphism ${\displaystyle \sigma ^{*}F\simeq p_{2}^{*}F}$ automatically satisfies the cocycle condition (this fact is noted at the end of the proof of Ch. 1, § 3., Proposition 1.5. of Mumford's "geometric invariant theory.")

If the action of G is free, then the notion of an equivariant sheaf simplifies to a sheaf on the quotient X/G, because of the descent along torsors.

By Yoneda's lemma, to give the structure of an equivariant sheaf to an ${\displaystyle {\mathcal {O}}_{X}}$-module F is the same as to give group homomorphisms for rings R over ${\displaystyle S}$,

${\displaystyle G(R)\to \operatorname {Aut} (X\times _{S}\operatorname {Spec} R,F\otimes _{S}R)}$.[3]

There is also a definition of equivariant sheaves in terms of simplicial sheaves. Alternatively, one can define an equivariant sheaf to be an equivariant object in the category of, say, coherent sheaves.

A structure of an equivariant sheaf on an invertible sheaf or a line bundle is also called a linearization.

Let X be a complete variety over an algebraically closed field acted by a connected reductive group G and L an invertible sheaf on it. If X is normal, then some tensor power ${\displaystyle L^{n}}$ of L is linearizable.[4]

Also, if L is very ample and linearized, then there is a G-linear closed immersion from X to ${\displaystyle \mathbf {P} ^{N}}$ such that ${\displaystyle {\mathcal {O}}_{\mathbf {P} ^{N}}(1)}$ is linearized and the linearlization on L is induced by that of ${\displaystyle {\mathcal {O}}_{\mathbf {P} ^{N}}(1)}$.[5]

Tensor products and the inverses of linearized invertible sheaves are again linearized in the natural way. Thus, the isomorphism classes of the linearized invertible sheaves on a scheme X form a subgroup of the Picard group of X.

See Example 2.16 of for an example of a variety for which most line bundles are not linearizable.

Given an algebraic group G and a G-equivariant sheaf F on X over a field k, let ${\displaystyle V=\Gamma (X,F)}$ be the space of global sections. It then admits the structure of a G-module; i.e., V is a linear representation of G as follows. Writing ${\displaystyle \sigma :G\times X\to X}$ for the group action, for each g in G and v in V, let

${\displaystyle \pi (g)v=(\varphi \circ \sigma ^{*})(v)(g^{-1})}$

where ${\displaystyle \sigma ^{*}:V\to \Gamma (G\times X,\sigma ^{*}F)}$ and ${\displaystyle \varphi$ :\Gamma (G\times X,\sigma ^{*}F){\overset {\sim }{\to }}\Gamma (G\times X,p_{2}^{*}F)=k[G]\otimes _{k}V} is the isomorphism given by the equivariant-sheaf structure on F. The cocycle condition then ensures that ${\displaystyle \pi :G\to GL(V)}$ is a group homomorphism (i.e., ${\displaystyle \pi }$ is a representation.)

Example: take ${\displaystyle X=G,F={\mathcal {O}}_{G}}$ and ${\displaystyle \sigma =}$ the action of G on itself. Then ${\displaystyle V=k[G]}$, ${\displaystyle (\varphi \circ \sigma ^{*})(f)(g,h)=f(gh)}$ and

${\displaystyle (\pi (g)f)(h)=f(g^{-1}h)}$,

meaning ${\displaystyle \pi }$ is the left regular representation of G.

The representation ${\displaystyle \pi }$ defined above is a rational representation: for each vector v in V, there is a finite-dimensional G-submodule of V that contains v.[6]

A definition is simpler for a vector bundle (i.e., a variety corresponding to a locally free sheaf of constant rank). We say a vector bundle E on an algebraic variety X acted by an algebraic group G is equivariant if G acts fiberwise: i.e., ${\displaystyle g:E_{x}\to E_{gx}}$ is a "linear" isomorphism of vector spaces.[7] In other words, an equivariant vector bundle is a pair consisting of a vector bundle and the lifting of the action ${\displaystyle G\times X\to X}$ to that of ${\displaystyle G\times E\to E}$ so that the projection ${\displaystyle E\to X}$ is equivariant.

Just like in the non-equivariant setting, one can define an equivariant characteristic class of an equivariant vector bundle.

• The tangent bundle of a manifold or a smooth variety is an equivariant vector bundle.
• The sheaf of equivariant differential forms.
• Let G be a semisimple algebraic group, and λ:H→C a character on a maximal torus H. It extends to a Borel subgroup λ:B→C, giving a one dimensional representation W_λ of B. Then GxW_λ is a trivial vector bundle over G on which B acts. The quotient L_λ=Gx_BW_λ by the action of B is a line bundle over the flag variety G/B. Note that G→G/B is a B bundle, so this is just an example of the associated bundle construction. The Borel-Weil-Bott theorem says that all representations of G arise as the cohomologies of such line bundles.
• If X=Spec(A) is an affine scheme, a G_m-action on X is the same thing as a Z grading on A. Similarly, a G_m equivariant quasicoherent sheaf on X is the same thing as a Z graded A module.

• Equivariant algebraic K-theory
• Equivariant bundle
• Equivariant cohomology
• Quotient stack

• J. Bernstein, V. Lunts, "Equivariant sheaves and functors," Springer Lecture Notes in Math. 1578 (1994).
• Mumford, David; Fogarty, J.; Kirwan, F. Geometric invariant theory. Third edition. Ergebnisse der Mathematik und ihrer Grenzgebiete (2) (Results in Mathematics and Related Areas (2)), 34. Springer-Verlag, Berlin, 1994. xiv+292 pp. MR1304906 ISBN 3-540-56963-4
• D. Gaitsgory, Geometric Representation theory, Math 267y, Fall 2005
• Thomason, R.W.:Algebraic K-theory of group scheme actions. In: Browder, W. (ed.) Algebraic topology and algebraic K-theory. (Ann. Math. Stud., vol. 113, pp. 539–563) Princeton: Princeton University Press 1987

• Equivariant sheaves