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Ieke Moerdijk

Publications and source records attributed to Ieke Moerdijk.

51 records · Page 3Linked to original sources
arXiv2007

A Unified Approach to Algebraic Set Theory

The paper provides an introduction to the field of Algebraic Set Theory (AST). AST is a flexible categorical framework for studying different kinds of set theories: both classical and constructive, predicative and impredicative. We discuss the basic results in this area, with a particular emphasis on applications to the constructive set theories IZF and CZF. (This paper is a summary of a tutorial on categorical logic given by the second named author at the Logic Colloquium 2006 in Nijmegen.)

math.LO↗
arXiv2007

On inner Kan complexes in the category of dendroidal sets

The category of dendroidal sets is an extension of that of simplicial sets, suitable for defining nerves of operads rather than just of categories. In this paper, we prove some basic properties of inner Kan complexes in the category of dendroidal sets. In particular, we extend fundamental results of Boardman and Vogt, of Cordier and Porter, and of Joyal to dendroidal sets.

math.AT↗
arXiv2007

Dendroidal Sets

We introduce the concept of a dendroidal set. This is a generalization of the notion of a simplicial set, specially suited to the study of operads in the context of homotopy theory. We define a category of trees, which extends the category $Δ$ used in simplicial sets, whose presheaf category is the category of dendroidal sets. We show that there is a closed monoidal structure on dendroidal sets which is closely related to the Boardman-Vogt tensor product of operads. Furthermore we show that each operad in a suitable model category has a coherent homotopy nerve which is a dendroidal set, extending another construction of Boardman and Vogt. There is also a notion of an inner Kan dendroidal set which is closely related to simplicial Kan complexes. Finally, we briefly indicate the theory of dendroidal objects and outline several of the applications and further theory of dendroidal sets.

math.AT↗
arXiv2006

The Boardman-Vogt resolution of operads in monoidal model categories

We extend the W-construction of Boardman and Vogt to operads of an arbitrary monoidal model category with suitable interval, and show that it provides a cofibrant resolution for well-pointed sigma-cofibrant operads. The standard simplicial resolution of Godement as well as the cobar-bar chain resolution are shown to be particular instances of this generalised W-construction.

math.AT↗
arXiv2003

Axiomatic homotopy theory for operads

We give sufficient conditions for the existence of a model structure on operads in an arbitrary symmetric monoidal model category. General invariance properties for homotopy algebras over operads are deduced.

math.AT↗
arXiv2003

The renormalisation bialgebra and operads

In a recent preprint, Brouder and Schmitt give a careful construction of a `renormalisation' Hopf algebra out of an arbitrary bialgebra. In this note, we point out that this is a special case of the construction of the cooperad of a bialgebra (Berger-Moerdijk) combined with the construction of a bialgebra from a cooperad (Frabetti-Van der Laan).

hep-th↗
arXiv2002

Orbifolds as Groupoids: an Introduction

This is a survey paper based on my talk at the Workshop on Orbifolds and String Theory, the goal of which was to explain the role of groupoids and their classifying spaces as a foundation for the theory of orbifolds.

math.DG↗
arXiv2000

Cech-De Rham theory for leaf spaces of foliations

The purpose of this paper is to present a ``Cech-De Rham'' model for the cohomology of leaf spaces. This model lends itself to the construction of characteristic classes (in the cohomology of classifying spaces) by explicit geometrical constructions which are immediate extensions of the standard constructions for manifolds. In particular we rediscover (and explain) the Thurston formula and the Bott formulas for cocycles on diffeomorphism groups. We also use the Cech-De Rham model to explicitly describe the relation between the cohomology of the classifying space, the basic cohomology, and the foliated cohomology of foliations.

math.DG↗
arXiv2000

On integrability of infinitesimal actions

We use foliations and connections on principal Lie groupoid bundles to prove various integrability results for Lie algebroids. In particular, we show, under quite general assumptions, that the semi-direct product associated to an infinitesimal action of one integrable Lie algebroid on another is integrable. This generalizes recent results of Dazord and Nistor.

math.DG↗
arXiv1999

A Homology Theory for Etale Groupoids

Etale groupoids arise naturally as models for leaf spaces of foliations, for orbifolds, and for orbit spaces of discrete group actions. In this paper we introduce a sheaf homology theory for etale groupoids. We prove its invariance under Morita equivalence, as well as Verdier duality between Haefliger cohomology and this homology. We also discuss the relation to the cyclic and Hochschild homologies of Connes' convolution algebra of the groupoid, and derive some spectral sequences which serve as a tool for the computation of these homologies.

math.KT↗
arXiv1997

simplicial cohomology of orbifolds

For any orbifold M, we explicitly construct a simplicial complex S(M) from a given triangulation of the `coarse' underlying space together with the local isotropy groups of M. We prove that, for any local system on M, this complex S(M) has the same cohomology as M. The use of S(M) in explicit calculations is illustrated in the example of the `teardrop' orbifold.

q-alg↗
arXiv1997

A definability theorem for first order logic

For any first order theory T we construct a Boolean valued model M, in which precisely the T--provable formulas hold, and in which every (Boolean valued) subset which is invariant under all automorphisms of M is definable by a first order formula. Our presentation is entirely selfcontained, and only requires familiarity with the most elementary properties of model theory.

math.LO↗