Higher-dimensional algebra

A first step towards defining higher dimensional algebras is the concept of 2-category of higher category theory, followed by the more 'geometric' concept of double category.

A higher level concept is thus defined as a category of categories, or super-category, which generalises to higher dimensions the notion of category – regarded as any structure which is an interpretation of Lawvere's axioms of the elementary theory of abstract categories (ETAC). Ll.

, Thus, a supercategory and also a super-category, can be regarded as natural extensions of the concepts of meta-category,multicategory, and multi-graph, k-partite graph, or colored graph (see a color figure, and also its definition in graph theory).

Supercategories were first introduced in 1970, and were subsequently developed for applications in theoretical physics (especially quantum field theory and topological quantum field theory) and mathematical biology or mathematical biophysics.

Other pathways in HDA involve: bicategories, homomorphisms of bicategories, variable categories (aka, indexed, or parametrized categories), topoi, effective descent, amnd enriched and internal categories.

In higher-dimensional algebra (HDA), a double groupoid is a generalisation of a one-dimensional groupoid to two dimensions, and the latter groupoid can be considered as a special case of a category with all invertible arrows, or morphisms.

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