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arXiv math/9906111math.ATmath.GR
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广义群上同调的Chern逼近

Chern approximations for generalised group cohomology

Neil P. Strickland

摘要:

设G是一个有限群,E是一个广义上同调理论,在一定的技术条件下.我们研究了一个环C(E,G),它是E^0BG的最佳可能逼近,可以只用G的复表示的知识来构建。存在一个自然映射C(E,G)-> E^0BG,它的像是E^0BG在E^0上由这类表示的所有陈类生成的子环。有充分的先例考虑这个子环在平行的情况下,普通上同调。然而,尽管这个子环的生成元来自于表示论,但对于关系却不能这样说;我们构造的目的之一就是为了弥补这一点。我们还开发了一种广义的字符理论,提供了良好的信息合理化的C(E,G)。在我们能够完全分析的少数情况下,要么C(E,G)由于简单的特征标理论原因而合理地不同于E^0BG,要么我们有C(E,G)=E^0BG。我们不直接研究环,而是研究形式概形X(G)=spf(E^0 BG)和XCh(G)=spf(C(E,G))。适当地解释,我们的主要定义是XCh(G)是从G的复表示的λ-半环R^+(G)到与E相关联的形式群上的因子的λ-半环概型的同态概型。

英文摘要:

Let G be a finite group, and let E be a generalised cohomology theory, subject to certain technical conditions. We study a certain ring C(E,G) that is the best possible approximation to E^0BG that can be built using only knowledge of the complex representations of G. There is a natural map C(E,G) -> E^0BG, whose image is the subring of E^0BG generated over E^0 by all Chern classes of such representations. There is ample precedent for considering this subring in the parallel case of ordinary cohomology. However, although the generators of this subring come from representation theory, the same cannot be said for the relations; one purpose of our construction is to remedy this. We also also develop a kind of generalised character theory which gives good information about the rationalisation of C(E,G). In the few cases that we have been able to analyse completely, either C(E,G) is rationally different from E^0BG for easy character-theoretic reasons, or we have C(E,G)=E^0BG. Rather than working directly with rings, we will study the formal schemes X(G)=spf(E^0BG) and XCh(G)=spf(C(E,G)). Suitably interpreted, our main definition is that XCh(G) is the scheme of homomorphisms from the Lambda-semiring R^+(G) of complex representations of G to the Lambda-semiring scheme of divisors on the formal group associated to E.

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