nLab Fox's theorem

Contents

Context

Monoidal categories

monoidal categories

With braiding

With duals for objects

With duals for morphisms

With traces

Closed structure

Special sorts of products

Semisimplicity

Morphisms

Internal monoids

Examples

Theorems

In higher category theory

Contents

Idea

A theorem due to Fox 1976 characterizes the cartesian products in a cartesian monoidal category as an algebraic structure given by natural transformations rather than in terms of a universal property.

Statement

Theorem

A symmetric monoidal category is cartesian if and only if it is isomorphic to its own category of cocommutative comonoids. Thus every object is equipped with a unique cocommutative comonoid structure, and these structures are respected by all maps.

Reference

The original article:

A survey of theorems extending Fox’s original theorem:

Last revised on August 18, 2025 at 09:27:04. See the history of this page for a list of all contributions to it.