The category of (left) πΊ-spaces\leftindex{G}\Set for a given groupπΊ
consists of sets (left-)acted on by πΊ (called a (left-)πΊ-space)
with πΊ-(left-)equivariant maps as morphisms. group
A (left-)equivariant map πβπΊ{(}π,π) is a function π:πβπ satisfying
π(πβ π₯)=πβ π(π₯)
for any π₯βπ and πβπΊ.
An isomorphism of πΊ-spaces is sometimes called an equivalence of actions.