Atiyah–Segal completion theorem
The Atiyah–Segal completion theorem is a theorem in mathematics about equivariant K-theory in homotopy theory. Let G be a compact Lie group and let X be a G-CW-complex. The theorem then states that the projection map
induces an isomorphism of prorings
In the special case of X a point, the theorem specializes to give an isomorphism between the K-theory of the classifying space of G and the completion of the representation ring.
The theorem can be interpreted as giving a comparison between the geometrical process of completing a G-space by making the action free and the algebraic process of completing with respect to an ideal. 
The theorem was first proved for finite groups by Michael Atiyah in 1961,  and a proof of the general case was published by Atiyah together with Graeme Segal in 1969.  Different proofs have since appeared generalizing the theorem to completion with respect to families of subgroups.   The corresponding statement for algebraic K-theory was proven by Merkujev, holding in the case that the group is algebraic over the complex numbers.
- Greenlees, J.P.C. (1996). "An introduction to equivariant K-theory.". CBMS Regional Conference Series. Equivariant homotopy and cohomology theory. Published for the Conference Board of the Mathematical Sciences, Washington, DC. pp. 143–152.
- Atiyah, M.F. (1961). "Characters and cohomology of finite groups" (PDF) 9 (1): 23–64. Retrieved 2008-06-19.
- Atiyah, M.F.; Segal, G.B. (1969). "Equivariant K-theory and completion" (PDF). Journal of Differential Geometry 3: 1–18. Retrieved 2008-06-19.
- Jackowski, S. (1985). "Families of subgroups and completion". J. Pure Appl. Algebra 37 (2): 167–179. doi:10.1016/0022-4049(85)90094-5.
- Adams, J.F.; Haeberly, J.P.; Jackowski, S.;; May, J.P. (1988). "A generalization of the Atiyah-Segal Completion Theorem". Topology 27 (1): 1–6. doi:10.1016/0040-9383(88)90002-X.
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