 Atiyah–Segal completion theorem

The Atiyah–Segal completion theorem is an important result in algebraic topology and representation theory, specifically in the context of stable homotopy theory and the study of equivariant stable homotopy types. In general, the theorem pertains to the completion of a space (or a category) in relation to certain types of groups (like finite groups), and it often deals with cohomology theories.