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Interchange law

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In mathematics, specifically category theory, the interchange law (or exchange law[1]) regards the relationship between vertical and horizontal compositions of natural transformations.

Let and where are functors and are categories. Also, let and while and where are natural transformations. For simplicity's and this article's sake, let and be the "secondary" natural transformations and and the "primary" natural transformations. Given the previously mentioned, we have the interchange law, which says that the horizontal composition () of the primary vertical composition () and the secondary vertical composition () is equal to the vertical composition () of each secondary-after-primary horizontal composition (); in short, .[2] It also appears in monoidal categories wherein classical composition () and the tensor product () take their places in lieu of the horizontal composition and vertical composition partnership and is denoted .[3]

The word "interchange" stems from the observation that the compositions and natural transformations on one side are switched or "interchanged" in comparison to the other side. The entire relationship can be shown within the following diagram.

Thumb
The interchange law in category theory.


If we apply this context to functor categories, and observe natural transformations and within a category and and within a category , we can imagine a functor , such that

the natural transformations are mapped like such:

  • and ;

functors are also mapped accordingly:

  • and .
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