categorical algebra
0.1 Introduction: An Outline of Categorical Algebra
This topic entry provides an outline of an important mathematical field called categorical algebra; although specific definitions are in use for various applications of categorical algebra to specific algebraic structures^{}, they do not cover the entire field. In the most general sense, categorical algebras– as introduced by Mac Lane in 1965 – can be described as the study of representations of algebraic structures, either concrete or abstract, in terms of categories^{}, functors^{} and natural transformations.
In a narrow sense, a categorical algebra is an associative algebra, defined for any locally finite category and a commutative ring with unity. This notion may be considered as a generalization^{} of both the concept of group algebra and that of an incidence algebra, much as the concept of category generalizes the notions of group and partially ordered set^{}.
0.2 Extensions of categorical algebra

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Thus, ultimately, since categories are interpretations^{} of the axiomatic elementary theory of abstract categories (ETAC), so are categorical algebras.
The general definition of representation introduced above can be still further extended by introducing supercategorical algebras^{} as interpretations of ETAS, as explained next.

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Mac Lane (1976) wrote in his Bull. AMS review cited here as a verbatim quotation:
“On some occasions I have been tempted to try to define what algebra is, can, or should be  most recently in concluding a survey [72] on Recent advances in algebra. But no such formal definitions hold valid for long, since algebra and its various subfields steadily change under the influence of ideas and problems coming not just from logic and geometry, but from analysis, other parts of mathematics, and extra mathematical sources. The progress of mathematics does indeed depend on many interlocking, unexpected and multiform developments.”
0.3 Basic definitions
An algebraic representation is generally defined as a morphism^{} $\rho $ from an abstract algebraic structure ${\mathrm{A}}_{S}$ to a concrete algebraic structure ${A}_{c}$, a Hilbert space $\mathscr{H}$, or a family of linear operator spaces.
The key notion of representable functor (http://planetmath.org/RepresentableFunctor) was first reported by Alexander Grothendieck in 1960.
Definition 0.1.
Thus, when the latter concept is extended to categorical algebra, one has a representable functor $S$ from an arbitrary category $\mathcal{C}$ to the category of sets $Set$ if $S$ admits a functor representation defined as follows. A functor representation of $S$ is defined as a pair, $(R,\varphi )$, which consists of an object $R$ of $\mathcal{C}$ and a family $\varphi $ of equivalences $\varphi (C):{\mathrm{Hom}}_{\mathcal{C}}(R,C)\cong S(C)$, which is natural in C, with C being any object in $\mathcal{C}$. When the functor $S$ has such a representation, it is also said to be represented by the object $R$ of $\mathcal{C}$. For each object $R$ of $\mathbf{C}$ one writes ${h}_{R}:\mathcal{C}\u27f6Set$ for the covariant $\mathrm{Hom}$–functor ${h}_{R}(C)\cong {\mathrm{Hom}}_{\mathcal{C}}(R,C)$. A representation $(R,\varphi )$ of $S$ is therefore a natural equivalence of functors:
$$\varphi :{h}_{R}\cong S.$$  (0.1) 
Remark 0.1.
The equivalence classes^{} of such functor representations (defined as natural equivalences) determine directly an algebraic (groupoid^{}) structure.
0.4 Note:
See also in Expositions the entry about abstract and concrete algebras.
0.5 Application: Quantum Categories
References
 1 Saunders Mac Lane: Categorical algebra., Bull. AMS, 71 (1965), 40106., Zbl 0161.01601, MR 0171826,
 2 Saunders Mac Lane: Topology^{} and Logic as a Source of Algebras., Bull. AMS, 82, Number 1, 136, January 1, 1976.
 3 http://en.wikipedia.org/wiki/Categorical_algebraCategorical algebra basic definitions
Title  categorical algebra 
Canonical name  CategoricalAlgebra 
Date of creation  20130322 18:13:27 
Last modified on  20130322 18:13:27 
Owner  bci1 (20947) 
Last modified by  bci1 (20947) 
Numerical id  79 
Author  bci1 (20947) 
Entry type  Topic 
Classification  msc 08A99 
Classification  msc 08A05 
Classification  msc 08A70 
Synonym  algebraic categories^{} 
Related topic  AlgebraicCategoryOfLMnLogicAlgebras 
Related topic  NonAbelianStructures 
Related topic  AbelianCategory 
Related topic  AxiomsForAnAbelianCategory 
Related topic  GeneralizedVanKampenTheoremsHigherDimensional 
Related topic  AxiomaticTheoryOfSupercategories 
Related topic  CategoricalOntology 
Related topic  NonCommutingGraphOfAGroup 
Related topic  NonAbelianStructures 
Defines  algebraic representation 
Defines  functor representation 
Defines  representable functor 
Defines  category of algebraic structures 
Defines  category of logic algebras 