# PlanetPhysics/Higher Dimensional Algebra 2

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### Basic Notion

Higher dimensional algebra (HDA) is a concept introduced by Ronald Brown to signify the extensions of various structures in algebraic topology and category theory to higher dimensions. Such extensions can be carried out in several possible ways, including also several published axiomatic approaches, ranging from ETAC to various ETAS axiom systems (e.g. the axiomatic theory of supercategories); these are currently being studied and improved upon. In Ronald Brown's own words, the HDA concept is generally defined, or understood, as follows.

### HDA Description

   "In general, Higher Dimensional Algebra (HDA) may be defined as the study of algebraic structures with operations whose domains of definitions are defined by geometric considerations. This allows for a splendid interplay of algebra and geometry, which early appeared in category theory with the use of complex commutative diagrams. What is needed next is a corresponding interplay with analysis and functional analysis that would extend also to quantum operator algebras, their representations and symmetries."
(quoted from R. Brown, 2008).

### HDA Examples

\htmladdnormallink{double groupoids {http://planetphysics.us/encyclopedia/WeakHomotopy.html}, double algebroids}, double and multiple categories.
Double groupoids are often used to capture information about geometrical objects such as higher-dimensional manifolds (or n-dimensional manifolds)[2]. In general, an n-dimensional manifold is a space that locally looks like an n-dimensional Euclidean space, but whose global structure may be non-Euclidean. A first step towards defining higher dimensional algebras is the concept of 2-category, followed by the more `geometric' concept of double category.
The geometry of squares and their compositions leads to a common representation of a double groupoid in the following form: \bigbreak
   Failed to parse (unknown function "\D"): {\displaystyle (1) \D= \vcenter{\xymatrix @=3pc {S \ar @<1ex> [r] ^{s^1} \ar @<-1ex> [r] _{t^1} \ar @<1ex> [d]^{\, t_2} \ar @<-1ex> [d]_{s_2} & H \ar[l] \ar @<1ex> [d]^{\,t} \ar @<-1ex> [d]_s \\ V \ar [u] \ar @<1ex> [r] ^s \ar @<-1ex> [r] _t & M \ar [l] \ar[u]}} }
where $M$ is a set of `points', $H,V$ are `horizontal' and `vertical' groupoids, and $S$ is a set of `squares' with two compositions. The laws for a double groupoid make it also describable as a groupoid internal to the category of groupoids.
Given two groupoids $H,V$ over a set $M$, there is a double groupoid $\Box (H,V)$ with $H,V$ as horizontal and vertical edge groupoids, and squares given by quadruples
   Failed to parse (unknown function "\begin{pmatrix}"): {\displaystyle \begin{pmatrix} & h& \<blockquote><math>-0.9ex] v & & v'\<blockquote><math>-0.9ex]& h'& \end{pmatrix} }
for which we assume always that $h,h'\in H,\,v,v'\in V$ and that the initial and final points of these edges match in $M$ as suggested by the notation, that is for example </math>sh=sv, th=sv', \ldots$,etc.Thecompositionsaretobeinheritedfromthoseof<math>H,V$, that is: \bigbreak
   Failed to parse (unknown function "\quadr"): {\displaystyle \quadr{h}{v}{v'}{h'} \circ_1\quadr{h'}{w}{w'}{h''} =\quadr{h}{vw}{v'w'}{h''}, \;\quadr{h}{v}{v'}{h'} \circ_2\quadr{k}{v'}{v''}{k'}=\quadr{hk}{v}{v''}{h'k'} ~. }
\bigbreak This construction is defined by the right adjoint \textsl{R} to the forgetful functor \textsl{L} which takes the double groupoid as above, to the pair of groupoids $(H,V)$ over $M$.
Remarks \\ Examples of contributions to HDA also include novel non--Abelian higher homotopy (and homology) results such as the outstanding extension provided by the higher dimensional, generalized Van Kampen theorems proved by Ronald Brown (,, and relevant references cited therein). Other examples are the concepts of R-Supercategory and 2-category of double groupoids.
Thus, several novel and important results pertinent to HDA were reported and/or published in the following areas: Algebraic Topology, higher dimensional Van Kampen theorems, supercategories, n-categories, double groupoids, double categories, double algebroids, and so on. Furthermore, both earlier and more recent HDA applications include: the developments in the axiomatic theory of supercategories, (ETAS; in refs. and), supercategories of complex systems, and Organismic Supercategories: superstructure and dynamics in Mathematical/theoretical Biology and Biophysics (,,,,). The interested reader is referred for further details to the following short bibliography list selected for this concise outline defining HDA.