Applied Categorical Structures

, Volume 3, Issue 4, pp 321–356 | Cite as

Trunks and classifying spaces

  • Roger Fenn
  • Colin Rourke
  • Brian Sanderson


Trunks are objects loosely analogous to categories. Like a category, a trunk has vertices and edges (analogous to objects and morphisms), but instead of composition (which can be regarded as given by preferred triangles of morphisms) it has preferred squares of edges. A trunk has a natural cubical nerve, analogous to the simplicial nerve of a category. The classifying space of the trunk is the realisation of this nerve. Trunks are important in the theory of racks [8]. A rackX gives rise to a trunkT (X) which has a single vertex and the setX as set of edges. Therack spaceBX ofX is the realisation of the nerveNT (X) ofT(X). The connection between the nerve of a trunk and the usual (cubical) nerve of a category determines in particular a natural mapBXBAs(X) whereBAs(X) is the classifying space of the associated group ofX. There is an extension to give a classifying space for an augmented rack, which has a natural map to the loop space of the Brown-Higgins classifying space of the associated crossed module [8, Section 2] and [3].

The theory can be used to define invariants of knots and links since any invariant of the rack space of the fundamental rack of a knot or link is ipso facto an invariant of the knot or link.

Mathematics Subject Classifications (1991)

57M 57Q 18C 18F 55R 55U 55P 

Key words

trunk classifying space rack cubical set nerve crossed module crossed complex knot link codimension 2 


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Copyright information

© Kluwer Academic Publishers 1995

Authors and Affiliations

  • Roger Fenn
    • 1
    • 2
  • Colin Rourke
    • 1
    • 2
  • Brian Sanderson
    • 1
    • 2
  1. 1.School of Mathematical SciencesUniversity of SussexBrightonU.K.
  2. 2.Mathematics InstituteUniversity of WarwickCoventryU.K.

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