434 research outputs found
Cluster structures for 2-Calabi-Yau categories and unipotent groups
We investigate cluster tilting objects (and subcategories) in triangulated
2-Calabi-Yau categories and related categories. In particular we construct a
new class of such categories related to preprojective algebras of non Dynkin
quivers associated with elements in the Coxeter group. This class of
2-Calabi-Yau categories contains the cluster categories and the stable
categories of preprojective algebras of Dynkin graphs as special cases. For
these 2-Calabi-Yau categories we construct cluster tilting objects associated
with each reduced expression. The associated quiver is described in terms of
the reduced expression. Motivated by the theory of cluster algebras, we
formulate the notions of (weak) cluster structure and substructure, and give
several illustrations of these concepts. We give applications to cluster
algebras and subcluster algebras related to unipotent groups, both in the
Dynkin and non Dynkin case.Comment: 49 pages. For the third version the presentation is revised,
especially Chapter III replaces the old Chapter III and I
Universal homogeneous constraint structures and the hom-equivalence classes of weakly oligomorphic structures
We derive a new sufficient condition for the existence of
{\omega}-categorical universal structures in classes of relational structures
with constraints, augmenting results by Cherlin, Shelah, Chi, and Hubi\v{c}ka
and Ne\v{s}et\v{r}il. Using this result we show that the hom-equivalence class
of any countable weakly oligomorphic structure has up to isomorphism a unique
model-complete smallest and greatest element, both of which are
{\omega}-categorical. As the main tool we introduce the category of constraint
structures, show the existence of universal homogeneous objects, and study
their automorphism groups. All constructions rest on a category-theoretic
version of Fra\"iss\'e's Theorem due to Droste and G\"obel. We derive
sufficient conditions for a comma category to contain a universal homogeneous
object. This research is motivated by the observation that all countable models
of the theory of a weakly oligomorphic structure are hom-equivalent---a result
akin to (part of) the Ryll-Nardzewski Theorem.Comment: 25 page
Restricted Priestley dualities and discriminator varieties
Anyone who has ever worked with a variety~ of
algebras with a reduct in the variety of bounded distributive lattices will
know a restricted Priestley duality when they meet one---but until now there
has been no abstract definition. Here we provide one. After deriving some basic
properties of a restricted Priestley dual category
of such a variety, we give a characterisation, in terms of
, of finitely generated discriminator subvarieties
of~.
As a first application of our characterisation, we give a new proof of
Sankappanavar's characterisation of finitely generated discriminator varieties
of distributive double p-algebras.
A substantial portion of the paper is devoted to the application of our
results to Cornish algebras. A Cornish algebra is a bounded distributive
lattice equipped with a family of unary operations each of which is either an
endomorphism or a dual endomorphism of the bounded lattice. They are a natural
generalisation of Ockham algebras, which have been extensively studied. We give
an external necessary-and-sufficient condition and an easily applied,
completely internal, sufficient condition for a finite set of finite Cornish
algebras to share a common ternary discriminator term and so generate a
discriminator variety. Our results give a characterisation of discriminator
varieties of Ockham algebras as a special case, thereby yielding Davey, Nguyen
and Pitkethly's characterisation of quasi-primal Ockham algebras
Metric abstract elementary classes as accessible categories
We show that metric abstract elementary classes (mAECs) are, in the sense of
[LR] (i.e. arXiv:1404.2528), coherent accessible categories with directed
colimits, with concrete -directed colimits and concrete
monomorphisms. More broadly, we define a notion of -concrete AEC---an
AEC-like category in which only the -directed colimits need be
concrete---and develop the theory of such categories, beginning with a
category-theoretic analogue of Shelah's Presentation Theorem and a proof of the
existence of an Ehrenfeucht-Mostowski functor in case the category is large.
For mAECs in particular, arguments refining those in [LR] yield a proof that
any categorical mAEC is -d-stable in many cardinals below the categoricity
cardinal.Comment: v2: changed terminology. v3: tightened inequalities. v4: clarifying
notes added. v5: referee's comments incorporated, with substantial
improvement
Multidimensional exact classes, smooth approximation and bounded 4-types
In connection with the work of Anscombe, Macpherson, Steinhorn and the
present author in [1] we investigate the notion of a multidimensional exact
class (-mec), a special kind of multidimensional asymptotic class (-mac)
with measuring functions that yield the exact sizes of definable sets, not just
approximations. We use results about smooth approximation [24] and Lie
coordinatisation [14] to prove the following result (Theorem 4.6.4), as
conjectured by Macpherson: For any countable language and any
positive integer the class of all finite
-structures with at most 4-types is a polynomial exact class
in , where a polynomial exact class is a multidimensional exact
class with polynomial measuring functions
Optical Substructures in 48 Galaxy Clusters
We analyze the presence of substructures in a set of 48 galaxy clusters, by
using galaxy positions and redshifts. We use a multi-scale analysis which
couples kinematical estimators with the wavelet transform. 14% of our clusters
are strongly substructured (i.e. they are bimodal or complex) and 24% of the
remaining unimodal clusters contain substructures at small scales. Thus, in
substantial agreement with previous studies, about one third of clusters show
substructures. In unimodal clusters the presence of substructures does not
affect the estimates of both virial masses and velocity dispersions, which are
generally in good agreement with the X-ray temperatures. Thus, unimodal
clusters are not too far from a status of dynamical equilibrium. On the
contrary, velocity dispersions and masses for some bimodal or complex clusters
strongly depend on whether they are treated as single systems or as sums of
different clumps and X-ray temperatures and velocity dispersions may be very
different.Comment: 24 pages, 5 bitmapped eps figures, 7 tables, USE LaTeX2e !!
Astrophysical Journal, in pres
Universal abstract elementary classes and locally multipresentable categories
We exhibit an equivalence between the model-theoretic framework of universal
classes and the category-theoretic framework of locally multipresentable
categories. We similarly give an equivalence between abstract elementary
classes (AECs) admitting intersections and locally polypresentable categories.
We use these results to shed light on Shelah's presentation theorem for AECs.Comment: 14 pages. Some typos remove
Higher-dimensional analogues of K3 surfaces
In the first part we survey some of the known results and conjectures on
compact Hyperkaehler (HK) manifolds. In the second part we presents a program
which aims to show that HK four-folds whose second cohomology (with 4-tuple
cup-product) is isomorphic to that of the Hilbert square of a K3 enjoy many of
the beautiful properties of K3 surfaces, in particular they should form a
single deformations class.Comment: The paper originated from a talk given at MSRI during the special
program in Algebraic Geometry (2009)
SUPPLEMENTED MORPHISMS
In the present paper, left R-modules M and N are studied
under the assumptions that HomR(M,N) is supplemented. It is shown
that Hom(M,N) is (⊕, G*, amply)-supplemented if and only if N is
(⊕, G*, amply)-supplemented. Some applications to cosemisimple modules,
refinable modules and UCC-modules are presented. Finally, the
relationship between the Jacobson radical J[M,N] of HomR(M,N) and
HomR(M,N) is supplemented are investigated. Let M be a finitely generated,
self-projective left R-module and N ∈ Gen(M). We show that if
Hom(M,N) is supplemented and N has GD2 then Hom(M,N)/J(M,N)
is semisimple as a left EM-module
Motivic toposes
We present a research programme aimed at constructing classifying toposes of
Weil-type cohomology theories and associated categories of motives, and
introduce a number of notions and preliminary results already obtained in this
direction. In order to analyze the properties of Weil-type cohomology theories
and their relations, we propose a framework based on atomic two-valued toposes
and homogeneous models. Lastly, we construct a syntactic triangulated category
whose dual maps to the derived categories of all the usual cohomology theories.Comment: 41 page
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