Contents

# Contents

## Idea

A coalgebra or comodule over a comonad $C$ on a category $A$ is an object $a\in A$ with a morphisms $a\to C a$ satisfying axioms formally dual to those for an algebra over a monad. The category of coalgebras is called its (co-)Eilenberg-Moore category and satisfies a universal property dual to that of the Eilenberg-Moore object? for a monad; it can thereby be internalized to any 2-category. The forgetful functor from the category of coalgebras to the category $A$ is called a comonadic functor. Similarly, a comonad also has a co-Kleisli category.

## References

Some introductory material on comonads, coalgebras and co-Kleisli morphisms can be found in

• Paolo Perrone, Notes on Category Theory with examples from basic mathematics, Chapter 5. (arXiv)

Last revised on January 14, 2020 at 23:45:24. See the history of this page for a list of all contributions to it.