Contents

categorification

# Contents

## Idea

Just as a monoidal category is the categorification of a monoid and a rig category is the categorification of a rig, a magmoidal category should be the categorification of a magma.

## Definition

A magmoidal category is a category $C$ with a functor

$\otimes: C \times C \to C$

from the product category of $C$ with itself.

If there is a specified object $I$, and natural isomorphisms $\lambda_A:\colon A\otimes I\stackrel{\simeq}{\to} A$ and $\rho_A:\colon I\otimes A\stackrel{\simeq}{\to} A$, then it is a magmoidal category with unit. Depending on the intended uses, it might be necessary to also impose the coherence condition that $\rho_I = \lambda_I\colon I\otimes I\stackrel{\simeq}{\to} I$ (which appeared in Mac Lane‘s original definition of a monoidal category, but proved redundant by Max Kelly). Note that the coherence condition usually placed on unitors in monoidal categories cannot even be stated in the context of a magmoidal category, since there is no associator to relate $A\otimes (I\otimes B)$ and $(A\otimes I)\otimes B$.

## Examples

For example, monoidal categories such as braided monoidal categories, symmetric monoidal categories, cartesian monoidal categories, cocartesian monoidal categories, and rig categories are all magma categories.

An Ackermann groupoid is a particular sort of poset that is magmoidal (where ‘groupoid’ is here a synonym for ‘magma’).

## Reference

The concept seems to first be named in

• Alexei Davydov, Nuclei of categories with tensor products, Theory and Applications of Categories, Vol. 18, 2007, No. 16, pp 440-472. tac:18-16

Last revised on May 16, 2022 at 02:10:53. See the history of this page for a list of all contributions to it.