Modification (category theory) Source: en.wikipedia.org/wiki/Modification_(category_theory)

In mathematics, specifically category theory, a modification is an arrow between natural transformations. It is a 3-cell in the 3-category of 2-cells (where the 2-cells are natural transformations, the 1-cells are functors, and the 0-cells are categories).^[1] The notion is due to Bénabou.^[2]

Given two natural transformations ${\displaystyle {\boldsymbol {\alpha ,\,\beta }}:{\boldsymbol {\mathbf {F} }}\rightarrow {\boldsymbol {\mathbf {G} }}}$, there exists a modification ${\displaystyle {\boldsymbol {\mathbf {\mu } }}:{\boldsymbol {\mathbf {\alpha } }}\rightarrow {\boldsymbol {\mathbf {\beta } }}}$ such that:

• ${\textstyle {\boldsymbol {\mathbf {\mu _{a}} }}:{\boldsymbol {\mathbf {\alpha _{a}} }}\rightarrow {\boldsymbol {\mathbf {\beta _{a}} }}}$,
• ${\textstyle {\boldsymbol {\mathbf {\mu _{b}} }}:{\boldsymbol {\mathbf {\alpha _{b}} }}\rightarrow {\boldsymbol {\mathbf {\beta _{b}} }}}$, and
• ${\textstyle {\boldsymbol {\mathbf {\mu _{f}} }}:{\boldsymbol {\mathbf {\alpha _{f}} }}\rightarrow {\boldsymbol {\mathbf {\beta _{f}} }}}$.^[1]

The following commutative diagram shows an example of a modification and its inner workings.

• Kelly, G. M.; Street, Ross (1974). "Review of the elements of 2-categories". In Kelly, Gregory M. (ed.). Category Seminar: Proceedings of the Sydney Category Theory Seminar, 1972/1973. Lecture Notes in Mathematics. Vol. 420. Springer. pp. 75–103. doi:10.1007/BFb0063101. ISBN 978-3-540-06966-9. MR 0357542.