# variable groupoid

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###### Definition 0.1.

A variable groupoid is defined as a family of groupoids $\{\mathsf{G}_{\lambda}\}$ indexed by a parameter $\lambda\in T$ , with $T$ being either an index set  or a class (which may be a time parameter, for time-dependent or dynamic groupoids      ). If $\lambda$ belongs to a set $M$, then we may consider simply a projection  $\mathsf{G}\times M{\longrightarrow}M$, which is an example of a trivial fibration  . More generally, one can consider a fibration of groupoids $\mathsf{G}\hookrightarrow Z{\longrightarrow}M$ (Higgins and Mackenzie, 1990) as defining a non-trivial variable groupoid.

Besides systems modelled in terms of a fibration of groupoids, one may consider a multiple groupoid defined as a set of $N$ groupoid structures, any distinct pair of which satisfy an interchange law which can be formulated as follows. There exists a unique expression with the following content:

 $\begin{bmatrix}x&y\\ z&w\end{bmatrix}\quad\objectmargin={0pt}\xy(0,4)*+{}="a",(0,-2)*+{\rule{0.0pt}% {6.45pt}i}="b",(7,4)*+{\;j}="c"\ar@{->}"a";"b"\ar@{->}"a";"c"\endxy,$ (0.1)

where $i$ and $j$ must be distinct for this concept to be well defined. This uniqueness can also be represented by the equation

 $(x\circ_{j}y)\circ_{i}(z\circ_{j}w)=(x\circ_{i}z)\circ_{j}(y\circ_{i}w).$ (0.2)

Brown and Higgins, 1981a, showed that certain multiple groupoids equipped with an extra structure called connections were equivalent     to another structure called a crossed complex which had already occurred in homotopy theory. such as double, or multiple groupoids (Brown, 2004; 2005). For example, the notion of an atlas of structures should, in principle, apply to a lot of interesting, topological and/or algebraic, structures: groupoids, multiple groupoids, Heyting algebras, $n$-valued logic algebras  and $C^{*}$-convolution -algebras   . Such examples occur frequently in Higher Dimensional Algebra  (HDA).

 Title variable groupoid Canonical name VariableGroupoid Date of creation 2013-03-22 18:15:45 Last modified on 2013-03-22 18:15:45 Owner bci1 (20947) Last modified by bci1 (20947) Numerical id 17 Author bci1 (20947) Entry type Definition Classification msc 55U05 Classification msc 55U35 Classification msc 55U40 Classification msc 18G55 Classification msc 18B40 Synonym variable topology Related topic VariableCategory Related topic HigherDimensionalAlgebra Related topic GroupoidCDynamicalSystem Related topic HDA Related topic VariableTopology Related topic HigherDimensionalAlgebraHDA Related topic Supercategories3 Related topic 2Category2 Defines family of groupoids Defines GroupoidCDynamicalSystem