## Generalized group characters and complex oriented cohomology theories

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- by Michael J. Hopkins, Nicholas J. Kuhn and Douglas C. Ravenel
- J. Amer. Math. Soc.
**13**(2000), 553-594 - DOI: https://doi.org/10.1090/S0894-0347-00-00332-5
- Published electronically: April 26, 2000
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## Abstract:

Let $BG$ be the classifying space of a finite group $G$. Given a multiplicative cohomology theory $E^{*}$, the assignment \[ G \longmapsto E^{*}(BG) \] is a functor from groups to rings, endowed with induction (transfer) maps. In this paper we investigate these functors for*complex oriented*cohomology theories $E^{*}$, using the theory of complex representations of finite groups as a model for what one would like to know. An analogue of Artin’s Theorem is proved for all complex oriented $E^*$: the abelian subgroups of $G$ serve as a detecting family for $E^*(BG)$, modulo torsion dividing the order of $G$. When $E^*$ is a complete local ring, with residue field of characteristic $p$ and associated formal group of height $n$, we construct a character ring of class functions that computes $\frac {1}{p}E^*(BG)$. The domain of the characters is $G_{n,p}$, the set of $n$–tuples of elements in $G$ each of which has order a power of $p$. A formula for induction is also found. The ideas we use are related to the Lubin–Tate theory of formal groups. The construction applies to many cohomology theories of current interest: completed versions of elliptic cohomology, $E_n^*$–theory, etc. The $n$th Morava K–theory Euler characteristic for $BG$ is computed to be the number of $G$–orbits in $G_{n,p}$. For various groups $G$, including all symmetric groups, we prove that $K(n)^*(BG)$ is concentrated in even degrees. Our results about $E^*(BG)$ extend to theorems about $E^*(EG\times _G X)$, where $X$ is a finite $G$–CW complex.

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## Bibliographic Information

**Michael J. Hopkins**- Affiliation: Department of Mathematics, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139
- Email: mjh@math.mit.edu
**Nicholas J. Kuhn**- Affiliation: Department of Mathematics, University of Virginia, Charlottesville, Virginia 22903
- Email: njk4x@virginia.edu
**Douglas C. Ravenel**- Affiliation: Department of Mathematics, University of Rochester, Rochester, New York 14627
- Email: drav@math.rochester.edu
- Received by editor(s): July 20, 1999
- Received by editor(s) in revised form: January 28, 2000
- Published electronically: April 26, 2000
- Additional Notes: All three authors were partially supported by the National Science Foundation.
- © Copyright 2000 American Mathematical Society
- Journal: J. Amer. Math. Soc.
**13**(2000), 553-594 - MSC (2000): Primary 55N22; Secondary 55N34, 55N91, 55R35, 57R85
- DOI: https://doi.org/10.1090/S0894-0347-00-00332-5
- MathSciNet review: 1758754