Minimal Geodesics and Nilpotent Fundamental Groups

Hedlund [18] constructed Riemannian metrics on n-tori, n 3 for which minimal geodesics are very rare. In this paper we construct similar examples for every nilpotent fundamental group. These examples show that Bangert's existence results of minimal geodesics [4] are optimal for nilpotent fundamental groups.     

Relative Group Cohomology and The Orbit Category

Let G be a finite group and ? be a family of subgroups of G closed under conjugation and taking subgroups. We consider the question whether there exists a periodic relative ?-projective resolution for ? when ? is the family of all subgroups H ≤ G with rk H ≤ rkG ? 1. We answer this question negatively by calculating the relative group cohomology ?H*(G, 𝔽₂) where G = ?/2 × ?/2 and ? is the family of cyclic subgroups of G. To do this calculation we first observe that the relative group cohomology ?H*(G, M) can be calculated using the ext-groups over the orbit category of G restricted to the family ?. In second part of the paper, we discuss the construction of a spectral sequence that converges to the cohomology of a group G and whose horizontal line at E ₂ page is isomorphic to the relative group cohomology of G.     

Finite Factor Representations of 2-Step Nilpotent Groups and the Orbit Method

In this paper, we describe factor representations of discrete 2-step nilpotent groups with 2-divisible center in the spirit of the orbit method. We show that some standard theorems of the orbit method are valid for these groups. In the case of countable 2-step nilpotent groups we explain how to construct a factor representation starting with an orbit of the “coadjoint representation.” We also prove that every factor representation (more precisely, every trace) can be obtained by this construction, and prove a theorem on the decomposition of a factor representation restricted to a subgroup. Bibliography: 7 titles. __________ Translated from Zapiski Nauchnykh Seminarov POMI, Vol. 307, 2004, pp. 120–140.     

Minimal Periodic Orbit Structure of 2-Dimensional Homeomorphisms

We present a method for estimating the minimal periodic orbit structure, the topological entropy, and a fat representative of the homeomorphism associated with the existence of a finite collection of periodic orbits of an orientation-preserving homeomorphism of the disk D². The method focuses on the concept of fold and recurrent bogus transition and is more direct than existing techniques. In particular, we introduce the notion of complexity to monitor the modification process used to obtain the desired goals. An algorithm implementing the procedure is described and some examples are presented at the end.     

Application of Nilpotent Approximation and the Orbit Method to the Search of the Diagonal Asymptotics of Sub-Riemannian Heat Kernels

Siberian Mathematical Journal - We propose a general scheme for the search of a fundamental solution to the hypoelliptic diffusion equation in a “sufficiently good” sub-Riemannian...     

Triple Cohomology and the Galois Cohomology of Profinite Groups

Given a cotriple 𝔾 = (G, ε, δ) on a category X and a functor E:X ^Opp→A into an abelian category A, there exists the cohomology theory of Barr and Beck: Hⁿ(X, E) ε |A| (n ≥ 0, X ε |X|), ([1], p.249). Almost all the important cohomology theories in mathematics have been shown to be special instances of such a general theory (see [1], [2] and [3]). Usually E arises from an abelian group object Y in X in the following manner: it is the contravariant functor from X into the category Ab of abelian groups that associates to each object X in X the abelian group X(X, Y) of maps from X to Y. In such a situation we shall write Hⁿ(X, Y)_𝔾 instead of Hⁿ(X, E)_G. Barr and Beck [2] have shown that the Eilenberg-MacLane cohomology groups H?ⁿ(π, A), n ≥ 2, can be re-captured as follows. One considers the free group cotriple 𝔾′ on the category Gps of groups, which induces in a natural manner a cotriple 𝔾 on the category (Gps, π) of groups over a fixed group π.     

Relative Cohomology and Generalized Tate Cohomology

We compare relative cohomology theories arising from using different proper resolutions of modules. Criteria for the vanishing of such distinctions are given in certain cases, and we show that this is related to the generalized Tate cohomology theory. We also demonstrate that the two balance properties admitted by the two different cohomology theories are actually equivalent in some cases. As applications, we recover many results obtained earlier in various contexts. At last we investigate derived functors with respect to the Auslander and Bass classes.     

一类幂零矩阵幂零指标的特征

设(L, ,·)是一个incline. 本文给出了一个incline上幂零矩阵幂零指标的特征.其结果改进了文[4]中的相应结论.     

Cohomology of the Grothendieck construction

We consider cohomology of small categories with coefficients in a natural system in the sense of Baues and Wirsching. For any functor L : K → CAT, we construct a spectral sequence abutting to the cohomology of the Grothendieck construction of L in terms of the cohomology of K and of L(k), for k ∈ ObK.The first author was supported by DFG at University of BielefeldThe second author is a researcher from CONICET, Argentina     

Cohomology of the Steenrod algebra

Translated from Matematicheskie Zametki, Vol. 52, No. 2, pp. 120–126, August, 1992.     

幂零Fuzzy矩阵的一些结论及幂零度的求法

本文给出计算幂零Fuzzy矩阵的幂零度的两个办法,同时给出一个有有向Hamilton路的有向图的一个充分条件.     

Cohomology of biquotients

Biquotients are non-homogeneous quotient spaces of Lie groups. Using the Serre spectral sequence and the method of Borel, we compute the cohomology algebra of these spaces in cases where the Lie group cohomology is not too complicated. Among these are the biquotients which are known to carry a metric of positive curvature.     

Cohomology of bifunctors

We initiate the study of the cohomology of (strict polynomial)bifunctors by introducing the foundational formalism, establishingnumerous properties in analogy with the cohomology of functors,and providing computational techniques. Since one of the initialmotivations for the study of functor cohomology was the determinationof H *(GL (k), S*(g) *(g)), we keep this challenging examplein mind as we achieve numerous computations which illustrateour methods.     

On the Dimension of Nilpotent Algebras

The Eggert conjecture claims that a finite commutative algebra R over a field of prime characteristic p has the property dim Rdim R⁽¹), where R(1) is the subspace of R spanned by the pth powers of elements of R. We obtain results related to this conjecture and results on nilpotent algebras of rather high nilpotency class.