Group-scheme action

In algebraic geometry, an action of a group scheme is a generalization of a group action to a group scheme. Precisely, given a group S-scheme G, a left action of G on an S-scheme X is an S-morphism

\sigma :G\times _{S}X\to X

such that

(associativity) $\sigma \circ (1_{G}\times \sigma )=\sigma \circ (m\times 1_{X})$ , where $m:G\times _{S}G\to G$ is the group law,
(unitality) $\sigma \circ (e\times 1_{X})=1_{X}$ , where $e:S\to G$ is the identity section of G.

A right action of G on X is defined analogously. A scheme equipped with a left or right action of a group scheme G is called a G-scheme. An equivariant morphism between G-schemes is a morphism of schemes that intertwines the respective G-actions.

More generally, one can also consider (at least some special case of) an action of a group functor: viewing G as a functor, an action is given as a natural transformation satisfying the conditions analogous to the above.^[1] Alternatively, some authors study group action in the language of a groupoid; a group-scheme action is then an example of a groupoid scheme.

^ In details, given a group-scheme action $\sigma$ , for each morphism $T\to S$ , $\sigma$ determines a group action $G(T)\times X(T)\to X(T)$ ; i.e., the group $G(T)$ acts on the set of T-points $X(T)$ . Conversely, if for each $T\to S$ , there is a group action $\sigma _{T}:G(T)\times X(T)\to X(T)$ and if those actions are compatible; i.e., they form a natural transformation, then, by the Yoneda lemma, they determine a group-scheme action $\sigma :G\times _{S}X\to X$ .

[1] In details, given a group-scheme action $\sigma$ , for each morphism $T\to S$ , $\sigma$ determines a group action $G(T)\times X(T)\to X(T)$ ; i.e., the group $G(T)$ acts on the set of T-points $X(T)$ . Conversely, if for each $T\to S$ , there is a group action $\sigma _{T}:G(T)\times X(T)\to X(T)$ and if those actions are compatible; i.e., they form a natural transformation, then, by the Yoneda lemma, they determine a group-scheme action $\sigma :G\times _{S}X\to X$ .

[1]