Spherical resolutions for compact Lie groups

Let G be a compact Lie group and A a G-space. When does there exist a relative G-CW-complex (X,A) with free G-action on X\A, such that X has the homology of a sphere? This paper gives sufficient conditions, which can be used for the construction of homotopy representations.     

Brauer groups and crepant resolutions

We suggest a twisted version of the categorical McKay correspondence and prove several results related to it.     

A Hall-Higman-Shult type theorem for arbitrary finite groups

We shall extend a fixed point theorem of Shult to arbitrary finite groups. This will have applications to the study of group automorphisms.     

Butler groups of arbitrary cardinality

We show that in the constructible universe, the two usual definitions of Butler groups are equivalent for groups of arbitrarily large power. We also prove that Bext²(G, T) vanishes for every torsion-free groupG and torsion groupT. Furthermore, balanced subgroups of completely decomposable groups are Butler groups. These results have been known, under CH, only for groups of cardinalities ≤ ℵ_ω. Partial support by NSF is gratefully acknowledged. Partially supported by U.S.-Israel Binational Science Foundation.     

Valuations on free resolutions and higher geometric invariants of groups

Dedicated to Beno Eckmann on the occasion of his seventieth birthday.     

Computing invariants of algebraic groups in arbitrary characteristic

Let G be an affine algebraic group acting on an affine variety X. We present an algorithm for computing generators of the invariant ring KG[X] in the case where G is reductive. Furthermore, we address the case where G is connected and unipotent, so the invariant ring need not be finitely generated. For this case, we develop an algorithm which computes KG[X] in terms of a so-called colon-operation. From this, generators of KG[X] can be obtained in finite time if it is finitely generated. Under the additional hypothesis that K[X] is factorial, we present an algorithm that finds a quasi-affine variety whose coordinate ring is KG[X]. Along the way, we develop some techniques for dealing with nonfinitely generated algebras. In particular, we introduce the finite generation ideal.     

The structure of symplectic groups over arbitrary commutative rings

LetR be an arbitrary commutative ring, andn be an integer ≥3. It is proved for any idealJ ofR thatESp _2n(R,J)=[ESp _2n(R),ESp _2n(J)]=[ESp _2n(R),ESp _2n(R,J)] =[ESp _2n(R),GSp _2n(R,J)]=[Sp _2n(R),ESp _2n(R,J)]. Furthermore, the problem of normal subgroups ofSp _2n(R) has an affirmative solution if and only ifaR=a ²R+2aR for eacha inR. This covers the relevant results of [4], [8], [10], [12] and [13]. Project Supported by the Science Fund of the Chinese Academy of Sciences