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2022, Cornell University - arXiv
In this follow-up to [16], we continue developing the notion of a lego category and its many applications to stratifiable spaces and the computation of their Grothendieck classes. We illustrate the effectiveness of this construction by giving very short derivations of the class of a quotient by the "stratified action" of a discrete group [1], the class of a crystallographic quotient, the class of both a polyhedral product and a polyhedral (or simplicial) configuration space [8], the class of a permutation product [19] and, foremost, the class of spaces of 0-cycles [11].
Journal of Combinatorial Theory, Series B, 1979
Previous work has recast the invariant theory of projective geometry in terms of first order logic. This approach is applied to two categories connected with combinatorial projective geometry and coordinatized combinatorial pregeometries to characterize those invariant formulas (capable of expressing geometric properties) in terms of the language of brackets or determinants. The axioms for the theory of coordinatized pregeometries in this language are presented and conclusions drawn about the significance of identities or syzygies in the study of combinatorial geometry.
K-theory, 1998
We give a unified approach to the Isomorphism Conjecture of Farrell and Jones on the algebraic Kand L-theory of integral group rings and to the Baum-Connes Conjecture on the topological K-theory of reduced C * -algebras of groups. The approach is through spectra over the orbit category of a discrete group G. We give several points of view on the assembly map for a family of subgroups and characterize such assembly maps by a universal property generalizing the results of Weiss and Williams to the equivariant setting. The main tools are spaces and spectra over a category and their associated generalized homology and cohomology theories, and homotopy limits.
Arxiv preprint arXiv:0901.0137, 2008
Let Γ q denote the q-th stage of the descending central series of the free group on n generators F n . For each q ≥ 2 and every topological group G, a simplicial space B * (q, G) is constructed where B n (q, G) = Hom(F n /Γ q , G) and the realizations B(q, G) = |B * (q, G)| filter the classifying space BG. In particular for q = 2 this yields a single space B(2, G) assembled from all the n-tuples of commuting elements in G. Homotopy properties of the B(q, G) are considered for finite groups, including their description as homotopy colimits. Cohomology calculations are provided for compact Lie groups. The spaces B(2, G) are described in detail for transitively commutative groups. Stable homotopy decompositions of the B(q, G) are also provided with a formula giving the cardinality of Hom(F n /Γ q , G) for finite discrete groups G (and thus the cardinality of Hom(Z n , G) in case q = 2) in terms of the ranks of the homology groups for the associated filtration quotients of B(q, G). Specific calculations for H 1 (B(q, G); Z) are shown to be delicate in case G is finite of odd order in the sense that resulting topological properties (which are not yet fully understood) are equivalent to the Feit-Thompson theorem.
Ars Mathematica Contemporanea, 2015
The regular objects in various categories, such as maps, hypermaps or covering spaces, can be identified with the normal subgroups N of a given group Γ, with quotient group isomorphic to Γ/N. It is shown how to enumerate such objects with a given finite automorphism group G, how to represent them all as quotients of a single regular object U(G), and how the outer automorphism group of Γ acts on them. Examples constructed include kaleidoscopic maps with trinity symmetry.
WSEAS transactions on information science and applications, 2023
We call classification the operation consisting of distributing objects in classes or groups which are, in general, less numerous than them. The relations between these classes may be (or not) partially or totally ordered. So there exist many kinds of classification schemes. Formally speaking, a classification may be a lattice, a semilattice, a chain, a hypergraph, a matroid, a tree, etc. Our purpose is to find the underlying mathematical structure of all these classifications. We explain how we can represent them in a unique way, constituting what has been called before a "metaclassification" and which is, in fact, a Chu space. Thus, category theory can describe in terms of morphisms different operations on classifications that, according to Barwise and Seligman, we report in the following. Finally, we show that such a "metaclassification" is the fundamental brick from which we can get some information about the mathematical continuum.
arXiv (Cornell University), 2022
We give a combinatorial description of shape theory using finite topological T 0-spaces (finite partially ordered sets). This description may lead to a sort of computational shape theory. Then we introduce the notion of core for inverse sequences of finite spaces and prove some properties.
2016
The aim of this paper is to compare stratifications of moduli spaces given by group actions in the case of similarity of matrices introduced by Arnold and the author's stratification by projective orbifolds, and its relation to deformations of elements in the moduli space.
We show that the category of orbits of the bounded derived category of a hereditary category under a well-behaved autoequivalence is canonically triangulated. This answers a question by A. Buan, R. Marsh and I. Reiten which appeared in their study with M. Reineke and G. Todorov of the link between tilting theory and cluster algebras (closely related to work by Caldero-Chapoton-Schiffler) and a question by H. Asashiba about orbit categories. We observe that the resulting triangulated orbit categories provide many easy examples of triangulated categories with the Calabi-Yau property. These include the category of projective modules over a preprojective algebra of generalized Dynkin type in the sense of Happel-Preiser-Ringel, whose triangulated structure goes back to Auslander-Reiten's work on the representation-theoretic approach to rational singularities.
Transactions of the American Mathematical Society, 1995
In this paper we have tried to reduce the classical classification problems for spaces and maps of the proper category and of the strong shape category to similar problems in the homotopy category of simplicial sets or in the homotopy category of simplicial M M -sets, which M M is the monoid of proper selfmaps of the discrete space N \mathbb {N} of nonnegative integers. Given a prospace (prosimplicial set) Y Y , we have constructed a simplicial set P ¯ R Y {\overline {\mathcal {P}} ^R}Y such that the Hurewicz homotopy groups of P ¯ R Y {\overline {\mathcal {P}} ^R}Y are the Grossman homotopy groups of Y Y . For the case of the end prospace Y = ε X Y = \varepsilon X of a space X X , we obtain Brown’s proper homotopy groups; and for the Vietoris prospace Y = V X Y = VX (introduced by Porter) of a compact metrisable space X X , we have Quigley’s inward groups. The simplicial subset P ¯ R Y {\overline {\mathcal {P}} ^R}Y of a tower Y Y contains, as a simplicial subset, the homotopy limi...
We present a beautiful interplay between combinatorial topology and homological algebra for a class of monoids that arise naturally in algebraic combinatorics. We explore several applications of this interplay. For instance, we provide a new interpretation of the Leray number of a clique complex in terms of non-commutative algebra.
2012
In a highly influential paper, Bidigare, Hanlon and Rockmore showed that a number of popular Markov chains are random walks on the faces of a hyperplane arrangement. Their analysis of these Markov chains took advantage of the monoid structure on the set of faces. This theory was later extended by Brown to a larger class of monoids called left regular bands. In both cases, the representation theory of these monoids played a prominent role. In particular, it was used to compute the spectrum of the transition operators of the Markov chains and to prove diagonalizability of the transition operators.
Algebra & Number Theory, 2013
In this paper we give a combinatorial view on the adjunction theory of toric varieties. Inspired by classical adjunction theory of polarized algebraic varieties we define two convex-geometric notions: the Q-codegree and the nef value of a rational polytope P . We define the adjoint polytope P (s) as the set of those points in P , whose lattice distance to every facet of P is at least s. We prove a structure theorem for lattice polytopes with high Q-codegree. If P (s) is empty for some s < 2/(dim P + 2), then P has lattice width one. This has consequences in Ehrhart theory and on polarized toric varieties with dual defect. We remark that polyhedral adjunction theory even works in cases where the canonical divisor is not Q-Cartier. Moreover, we illustrate how classification results in adjunction theory can be translated into new classification results for lattice polytopes.
Journal of the London Mathematical Society, 2007
The concept of generalised species of structures between small categories and, correspondingly, that of generalised analytic functor between presheaf categories are introduced. An operation of substitution for generalised species, which is the counterpart to the composition of generalised analytic functors, is also put forward. These definitions encompass most notions of combinatorial species considered in the literature-including of course Joyal's original notion-together with their associated substitution operation. Our first main result exhibits the substitution calculus of generalised species as arising from a Kleisli bicategory for a pseudo-comonad on profunctors. Our second main result establishes that the bicategory of generalised species of structures is cartesian closed.
arXiv: Group Theory, 2018
This thesis takes Brady's construction of $K(\pi,1)$s for the braid groups as a starting point. It is widely known that this construction can - with the right ingredients - be generalized to Artin groups of finite type. Results of Bessis as well as Brady and Watt are used to establish the general construction for Artin groups of finite type. The non-crossing partition lattice in finite Coxeter groups is identified and used to generate the so called poset group. With the help of this poset group, which turns out to be isomorphic to the Artin group, a simplicial complex is constructed on which the poset group acts. It is shown that the complex itself is the universal cover of the $K(\pi,1)$ of the Artin group of finite type and the quotient under the action is the desired $K(\pi,1)$.
Configuration Spaces, 2012
In the first part we review some topological and algebraic aspects in the theory of Artin and Coxeter groups, both in the finite and infinite case (but still, finitely generated). In the following parts, among other things, we compute the Schwartz genus of the covering associated to the orbit space for all affine Artin groups. We also give a partial computation of the cohomology of the braid group with non-abelian coefficients coming from geometric representations. We introduce an interesting class of "sheaves over posets", which we call "weighted sheaves over posets", and use them for explicit computations.
Transformation Groups, 2011
Let G be a finite group. Given a finite G-set X and a modular tensor category C, we construct a weak G-equivariant fusion category C X , called the permutation equivariant tensor category. The construction is geometric and uses the formalism of modular functors. As an application, we concretely work out a complete set of structure morphisms for Z/2permutation equivariant categories, finishing thereby a program we initiated in [BFRS10]. * Email addresses:
1998
Categories are known to be useful for organizing structural aspects of mathematics. However, they are also useful in finding out what structure can be dismissed (coherence theorems) and hence in aiding calculations. We want to illustrate this for finite set theory, linear algebra, and group representation theory. We begin with some combinatorial set theory. Let N denote the set of natural numbers. We identify each n⁄⁄Œ⁄⁄N with the finite set n = { j⁄⁄Œ⁄⁄N: 0 £ j < n}. However, we must be careful to distinguish the cartesian product m ⁄⁄ ¥ ⁄⁄n = { (i⁄⁄,⁄⁄j) : 0 £ i < m, 0 £ j < n} from the isomorphic set mn. Let S denote the skeletal category of finite sets; explicitly, the objects are the n⁄⁄Œ⁄⁄N and the morphisms are the functions between these sets. We need to discuss the explicit construction of finite products in S⁄⁄. Let p 0 m n, : mn aAm and p1 m n, : mn aAn be the functions given by p 0 m n, (k) = i and p1 m n, (k) = j where k = i ⁄⁄n + j. That p 0 m n, and p1 m n, a...
Advances in Mathematics, 2007
Generalising Segal's approach to 1-fold loop spaces, the homotopy theory of n-fold loop spaces is shown to be equivalent to the homotopy theory of reduced Θ n-spaces, where Θ n is an iterated wreath product of the simplex category Δ. A sequence of functors from Θ n to Γ allows for an alternative description of the Segal spectrum associated to a Γ-space. This yields a canonical reduced Θ n-set model for each Eilenberg-MacLane space. The number of (n + k)-dimensional cells of the resulting CW-complex of type K(Z/2Z, n) is the kth generalised Fibonacci number of order n.
Israel Journal of Mathematics, 2011
In this paper we obtain a description of the BZ/p-cellularization (in the sense of Dror-Farjoun) of the classifying spaces of all finite groups, for all primes p.
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