Diagonal p-permutation functors

In [7] we have introduced the Frobenius categories [Formula: see text] over a finite p-group P, and we have associated to [Formula: see text] — suitably endowed with some central k*-extensions — a “Grothendieck group” as an inverse limit of Grothendieck groups of categories of modules in characteristic p obtained from [Formula: see text], determining its rank. Our purpose here is to introduce an analogous inverse limit of Grothendieck groups of categories of modules in characteristic zero obtained from [Formula: see text], determining its rank and proving that its extension to a field is canonically isomorphic to the direct sum of the corresponding extensions of the “Grothendieck groups” above associated with the centralizers in [Formula: see text] of a suitable set of representatives of the [Formula: see text]-classes of elements of P.

The Grothendieck ring of a finite group

For a commutative ring A we study here the Grothendieck group, K0A, of finitely generated projective A-modules, and the Picard group, Pic (A), of projective modules of rank one (under (?A). Writing koA for the elements of rank zero in KOA, there is an epimorphism det: K0A Pic (A) defined by the rth exterior power of a projective module of rank r. Our original objective was to calculate KJ(Zw), where w is a finitely generated abelian group. We do this in ? 8. The bulk of the paper is a rather general discussion of the techniques, some already familiar in other contexts, which we use to make this calculation. Our procedure is to decompose w as w0 x T, with w0 a finite group and T free abelian, and then to write Zw= A[T] where A = Zw0. Then A is a noetherian ring of (Krull) dimension one. For such an A we show (Theorem 7.8) that det: ko(A[T]) Pic (A[T]) is an isomorphism. In ?9 we prove also a non-stable analogue of this for T of rank <2. In ?8 we show (Theorem 8.1) that

In their 1988 paper ‘Gluing of perverse sheaves and discrete series representations’, D. Kazhdan and G. Laumon constructed an abelian category $\mathcal{A}$ associated to a reductive group G over a finite field with the aim of using it to construct discrete series representations of the finite Chevalley group $G(\mathbb{F}_q)$ . The well-definedness of their construction depended on their conjecture that this category has finite cohomological dimension. This was disproved in 2001 by R. Bezrukavnikov and A. Polishchuk, who found a counterexample in the case $G = SL_3$ . Polishchuk then made an alternative conjecture: though this counterexample shows that the Grothendieck group $K_0(\mathcal{A})$ is not spanned by objects of finite projective dimension, he noted that a graded version of $K_0(\mathcal{A})$ can be thought of as a module over Laurent polynomials and conjectured that a certain localization of this module is generated by objects of finite projective dimension, and suggested that this conjecture could lead toward a proof that Kazhdan and Laumon’s construction is well defined. He proved this conjecture in Types $A_1, A_2, A_3$ , and $B_2$ . In the present paper, we prove Polishchuk’s conjecture for all types, and prove that Kazhdan and Laumon’s construction is indeed well defined, giving a new geometric construction of discrete series representations of $G(\mathbb{F}_q)$ .

The group D(P) of all endo-permutation modules for a finite p-group P is a finitely generated abelian group. We prove that its torsion-free rank is equal to the number of conjugacy classes of non-cyclic subgroups of P. We also obtain partial results on its torsion subgroup. We determine next the structure of Q\otimes D(-) viewed as a functor, which turns out to be a simple functor S_{E,Q}, indexed by the elementary group E of order p^2 and the trivial Out(E)-module Q. Finally we describe a rather strange exact sequence relating Q\otimes D(P), Q\otimes B(P), and Q\otimes R(P), where B(P) is the Burnside ring and R(P) is the Grothendieck ring of QP-modules.

A note on Grothendieck groups of periodic derived categories

Let X, Y be compact, connected, metrisable spaces with base points Xo, Yo and let denote the compact operators. It is shown that Co(X\xo)| is asymptotically homotopic (or shape equivalent) to Co(Y~yo) @ Yi r if and only if X and Y have isomorphic K-groups. Similar results are obtained for certain inductive limits of nuclear C*-algebras. Let ~r denote the category whose objects are all the separable C*-algebras and whose set of morphisms from A to B, denoted [[A, B]], consists of homotopy classes of asymptotic morphisms. The construction of this category is due to Connes and Higson [CH], who defined a bivariant homology theory E(A, B) = lISA, SB | ~ff]] and have shown how to define the intersection product for arbitrary extensions of separable C*-algebras. If A is K-nuclear then E-theory agrees with Kasparov's bivariant K-theory [K]. On the other hand the asymptotic homotopy category d appears to be the "right" framework for the homotopy theory of separable C*-algebras. This point of view is supported by results in [H; CH; D; CHI; CK; D1]. For instance we have shown in [D1] that asymptotic homotopy is equivalent to a strong shape theory and hence is intimately related to the shape theories of [EK1] and [B] which were intended as homotopy theories for noncommutative singular spaces. In particular it turned out that two separable C*-algebras are shape equivalent if and only if they are asymptotically homotopic i.e. isomorphic in ~r The isomorphism class in sr of a separable C*-algebra A is called the asymptotic homotopy type of A. In this note we exhibit large classes of (projeetionless) stable, nuclear C*algebras whose asymptotic homotopy type is determined by K-theoretical data (Theorem 6). This is done via a suspension isomorphism

Suppose G is a finite group acting on a projective scheme X over a commutative Noetherian ring R. We study the RG-modules H 0 (X,F ⊗ L n ) when n > 0, and F and £ are coherent G-sheaves on X such that L is an ample line bundle. We show that the classes of these modules in the Grothendieck group G 0 (RG) of all finitely generated RG-modules lie in a finitely generated subgroup. Under various hypotheses, we show that there is a finite set of indecomposable RG-modules such that each H 0 (X, F⊗L n ) is a direct sum of these indecomposables, with multiplicities given by generalized Hilbert polynomials for n » 0.

Grothendieck groups of subcategories

By an Alperin group we mean a group in which the commutant of each 2-generated subgroup is cyclic. Alperin proved that if p is an odd prime then all finite p-groups with this property are metabelian. The today’s actual problem is the construction of examples of nonmetabelian finite Alperin 2-groups. Note that the author had given some examples of finite Alperin 2-groups with second commutants isomorphic to Z 2 and Z 4 and proved the existence of finite Alperin 2-groups with cyclic second commutants of however large order by appropriate examples. In this article the existence is proved of finite Alperin 2-groups with abelian second commutants of however large rank.

A restriction on centralizers in finite groups

A ring is right finite dimensional if it contains no infinite direct sum of right ideals.We prove that if a group G is finite, free abelian, or finitely generated abelian, then a ring R is right finite dimensional if and only if the group ring RG is right finite dimensional.A ring R is a self-injective cogenerator ring if Rn is injective and RR is a cogenerator in the category of unital right /{-modules; this means that each right unital -module can be embedded in a direct product of copies of R. Let G be a finite group where the order of G is a unit in R. Then the group ring RG is a selfinjective cogenerator ring if and only if R is a self-injective cogenerator ring.Additional applications are given.

It is a known fact that the subgroup Ω2(G) generated by all elements of order at most 4 in a finite 2-group G has a strong influence on the structure of the whole group G. For example, if Ω2(G) is metacyclic, then G is also metacyclic (N. Blackburn). Here we consider the case Ω2(G) = C2 x D, where C2 is cyclic of order 2 and D is any 2-group of maximal class and we show that ❘G : Ω2(G)❘ ≤ 2 and the structure of G is uniquely determined. We determine also the structure of a finite 2-group G whose elements of order 4 generate the subgroup Ω*2 (G) ≅ C2 × Q2n, where Q2n is generalized quaternion of order 2n. Finally, we show that a finite p-group G all of whose non-cyclic subgroups are generated by elements of order p is cyclic or of exponent p or a dihedral 2-group.

Abelian torsion-free groups of finite rank with finite automorphism groups are considered as rigid extensions of a system of strongly indecomposable groups Aj, j = 1, ..., k, of finite rank and having finite automorphism groups, by a finite p-group P. Such groups are called (A, p)-groups. The author introduces for (A, P)-groups the concept of (A, P)-type, which represents a choice of k integer matrices. A complete description of (A, P)-groups is given by means of (A, P)-types. Using this description, a series of problems on finite groups of automorphisms of torsion-free abelian groups of finite rank are solved. Furthermore, it is shown that the actual solution of any one of these problems comes down to a question of the consistency of a system of equations of the first degree modulo pt, where pt is the maximal order of elements of P. Bibliography: 11 titles.

Let G be a finite group. The prime graph Γ(G) of G is defined as follows. The vertices of Γ(G) are the primes dividing the order of G and two distinct vertices p and p′ are joined by an edge if there is an element in G of order pp′. We denote by k(Γ(G)) the number of isomorphism classes of finite groups H satisfying Γ(G) = Γ(H). Given a natural number r, a finite group G is called r-recognizable by prime graph if k(Γ(G)) = r. In Shen et al. (Sib. Math. J. 51(2):244–254, 2010), it is proved that if p is an odd prime, then B p (3) is recognizable by element orders. In this paper as the main result, we show that if G is a finite group such that Γ(G) = Γ(B p (3)), where p > 3 is an odd prime, then \({G\cong B_p(3)}\) or C p (3). Also if Γ(G) = Γ(B 3(3)), then \({G\cong B_3(3), C_3(3), D_4(3)}\), or \({G/O_2(G)\cong {\rm Aut}(^2B_2(8))}\). As a corollary, the main result of the above paper is obtained.