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Associativity isomorphism
Concept from category theory From Wikipedia, the free encyclopedia
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In mathematics, specifically in the field of category theory, the associativity isomorphism implements the notion of associativity with respect to monoidal products in semi-groupal (or monoidal-without-unit) categories.
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Definition
A category, , is called semi-groupal if it comes equipped with a functor such that the pair for , as well as a collection of natural isomorphisms known as the associativity isomorphisms (or "associators").[1][2][full citation needed] These isomorphisms, , are such that the following "pentagon identity" diagram commutes.

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Applications
In tensor categories
A tensor category,[3][full citation needed] or monoidal category, is a semi-groupal category with an identity object, , such that and . modular tensor categories have many applications in physics,[speculation?] especially in the field of topological quantum field theories.[4][unreliable source?][5][dubious – discuss]
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References
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