Interchange law

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

Let ${\textstyle \mathbf {F,\,G,\,H} :\mathbb {C} \longrightarrow \mathbb {D} }$ and ${\textstyle \mathbf {{\bar {F}},\,{\bar {G}},\,{\bar {H}}} :\mathbb {D} \longrightarrow \mathbb {E} }$ where ${\textstyle \mathbf {F,\,G,\,H,\,{\bar {F}},\,{\bar {G}},\,{\bar {H}}} }$ are functors and ${\textstyle \mathbb {C} ,\,\mathbb {D} ,\,\mathbb {E} }$ are categories. Also, let ${\displaystyle {\boldsymbol {\alpha }}:\mathbf {F\longrightarrow G} }$ and ${\displaystyle {\boldsymbol {\beta }}:\mathbf {G\longrightarrow H} }$ while ${\displaystyle {\boldsymbol {\bar {\alpha }}}:\mathbf {{\bar {F}}\longrightarrow {\bar {G}}} }$ and ${\displaystyle {\boldsymbol {\bar {\beta }}}:\mathbf {{\bar {G}}\longrightarrow {\bar {H}}} }$ where ${\displaystyle {\boldsymbol {\alpha }},\,{\boldsymbol {\beta }},\,{\boldsymbol {\bar {\alpha }}},\,{\boldsymbol {\bar {\beta }}}}$ are natural transformations. For simplicity's and this article's sake, let ${\displaystyle {\boldsymbol {\bar {\alpha }}}}$ and ${\displaystyle {\boldsymbol {\bar {\beta }}}}$ be the "secondary" natural transformations and ${\displaystyle {\boldsymbol {\alpha }}}$ and ${\displaystyle {\boldsymbol {\beta }}}$ the "primary" natural transformations. Given the previously mentioned, we have the interchange law, which says that the horizontal composition (${\displaystyle \circ }$) of the primary vertical composition (${\displaystyle \bullet }$) and the secondary vertical composition (${\displaystyle \bullet }$) is equal to the vertical composition (${\displaystyle \bullet }$) of each secondary-after-primary horizontal composition (${\displaystyle \circ }$); in short, ${\textstyle ({\bar {\boldsymbol {\beta }}}\ \bullet \ {\bar {\boldsymbol {\alpha }}})\circ \ ({\boldsymbol {\beta }}\ \bullet \ {\boldsymbol {\alpha }})\ =({\bar {\boldsymbol {\beta }}}\ \circ \ {\boldsymbol {\beta }})\ \bullet \ ({\bar {\boldsymbol {\alpha }}}\ \circ \ {\boldsymbol {\alpha }})}$.^[2] It also appears in monoidal categories wherein classical composition (${\displaystyle \circ }$) and the tensor product (${\displaystyle \otimes }$) take their places in lieu of the horizontal composition and vertical composition partnership and is denoted ${\displaystyle (g\circ f)\otimes (j\circ h)=(g\otimes j)\circ (f\otimes h)}$.^[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.

If we apply this context to functor categories, and observe natural transformations ${\displaystyle {\boldsymbol {\alpha }}:\mathbf {F\longrightarrow G} }$ and ${\displaystyle {\boldsymbol {\beta }}:\mathbf {G\longrightarrow H} }$ within a category ${\displaystyle V}$ and ${\displaystyle {\boldsymbol {\bar {\alpha }}}:\mathbf {{\bar {F}}\longrightarrow {\bar {G}}} }$ and ${\displaystyle {\boldsymbol {\bar {\beta }}}:\mathbf {{\bar {G}}\longrightarrow {\bar {H}}} }$ within a category ${\displaystyle W}$, we can imagine a functor ${\displaystyle \Gamma :V\longrightarrow W}$, such that

the natural transformations are mapped like such:

• ${\displaystyle \Gamma ({\boldsymbol {\alpha }})\longrightarrow {\boldsymbol {\bar {\alpha }}},\,}$
• ${\displaystyle \Gamma ({\boldsymbol {\beta }})\longrightarrow {\boldsymbol {\bar {\beta }}},\,}$
• and ${\displaystyle \Gamma ({\boldsymbol {\beta }}\ \bullet \ {\boldsymbol {\alpha }})\longrightarrow ({\boldsymbol {\bar {\beta }}}\ \bullet \ {\boldsymbol {\bar {\alpha }}})}$;

functors are also mapped accordingly:

• ${\displaystyle \Gamma ({\boldsymbol {\mathbf {F} }})\longrightarrow ({\boldsymbol {\mathbf {\bar {F}} }}),\,}$
• ${\displaystyle \Gamma ({\boldsymbol {\mathbf {G} }})\longrightarrow ({\boldsymbol {\mathbf {\bar {G}} }}),\,}$
• and ${\displaystyle \Gamma ({\boldsymbol {\mathbf {H} }})\longrightarrow ({\boldsymbol {\mathbf {\bar {H}} }})}$.