In a cubical set, you are guaranteed for every -cell (which can be drawn as a 1-cell)
that there is the identity -cell (which can be drawn as a 2-cell) of the form
A cubical set is said to have connections if in addition it has for every -cell also -cells of the form
And so forth. You should think of this as saying that the “thin” cell
is regarded as a degenerate cube by the cubical set in all the possible ways.
So it’s a very natural condition, particularly if you think of all these cubical cells as cubical paths in some space.
If is a cubical set, then a connection structure on consists of functions , , satisfying the relations for :
The connections are to be thought of as “extra degeneracies”. A degenerate cube of type has opposite faces equal and all other faces degenerate.
A cube of type has a pair of adjacent faces equal and all other faces of type or . So this makes the cubical theory nearer to the simplicial. Cubical complexes with this, and other, structures have also been considered by Evrard.
The first appearance of this notion in dimension was in the paper by Brown and Spencer listed below, and used to obtain an equivalence between crossed modules and edge symmetric double groupoids with connection.
Such connections on cubical sets were introduced in 1981 by Brown and Higgins in order to obtain the equivalence of their “cubical ω-groupoids” with crossed complexes. They are also essential to allow the notion of “commutative -shell” in such a structure.
But the category of cubes with connection is even a strict test category (Maltsiniotis, 2008). This means that under geometric realization (see the discussion at homotopy hypothesis) the cartesian product of cubical sets with connection is sent to the correct product homotopy type.
The lack of this property for cubical sets without connection was one of the original reasons reasons for abandoning Kan’s initial cubical approach to combinatorial homotopy theory in favour of the simplicial approach; the implications of this new result have yet to be thought through. Another reason was that cubical groups were in general not Kan complexes; however cubical groups with connection are Kan complexes. See the paper by Tonks listed below.
The prime example of a cubical set with connections is the singular cubical complex of a topological space . Here for is the set of singular -cubes in (i.e. continuous maps ) and the connection is induced by the map defined by
where as respectively.
The first hint of such a general structure came in the paper by Brown and Spencer given below. The term “connection” was used there because of a relation of a generalisation of this idea to path-connections in differential geometry. A principal -bundle over gives rise to the Ehresmann groupoid of -maps between the fibres, and the Moore paths on this form a double category with and as edge categories. A connection is then a functor from to one of the category structures on which gives a smooth lifting of paths to transport of the fibres. This is the origin of the term transport law? for the relation of connections to composition.
Ronnie Brown and C.B. Spencer, “Double groupoids and crossed modules’’, Cah. Top. Géom. Diff. 17 (1976) 343–362.
Evrard, M., “Homotopie des complexes simpliciaux et cubiques”, Preprint(1976).
Brown, R. and Higgins, P.J., “On the algebra of cubes”, J. Pure Appl. Algebra 21 (1981) 233–260.
F. Al-Agl, R. Brown and R. Steiner, “Multiple categories: the equivalence between a globular and cubical approach”, Advances in Mathematics, 170 (2002), 71–118.
M. Grandis and L. Mauri, “Cubical sets and their site”, Theory Applic. Categories, 11 (2003) 185–201.
P.J. Higgins, “Thin elements and commutative shells in cubical -categories”, Theory Appl. Categ. 14 (2005) 60–74.
The statement that cubical groups with connections are Kan complexes is due to
Cubical sets with connection are used in the following paper:
and in the following in preference to simplicial methods:
The statement that cubes with connection form a strict test category is due to