descent along torsors

{{Short description|Algebraic geometry}}

In mathematics, given a G-torsor X → Y and a stack F, the descent along torsors says there is a canonical equivalence between F(Y), the category of Y-points and F(X)^G, the category of G-equivariant X-points.{{harvnb|Vistoli|2008|loc=Theorem 4.46}} It is a basic example of descent, since it says the "equivariant data" (which is an additional data) allows one to "descend" from X to Y.

When G is the Galois group of a finite Galois extension L/K, for the G-torsor $\backslash operatorname\{Spec\}\; L\; \backslash to\; \backslash operatorname\{Spec\}\; K$, this generalizes classical Galois descent (cf. field of definition).

For example, one can take F to be the stack of quasi-coherent sheaves (in an appropriate topology). Then F(X)^G consists of equivariant sheaves on X; thus, the descent in this case says that to give an equivariant sheaf on X is to give a sheaf on the quotient X/G.