free completion

For a small category $S$, just as the presheaf category $Set^{S^{op}}$ is the free cocompletion of $S$, by duality we can conclude that $(Set^S)^{op}$ is the **free completion** of $S$. This means that any functor $S\to C$ where $C$ is complete factors uniquely (up to isomorphism) through the “dual Yoneda embedding” $S\to (Set^S)^{op}$ via a continuous functor $(Set^S)^{op}\to C$.

This operation is a 2-monad which is lax idempotent but not (even weakly) idempotent. In analogy with the general concept of completion, we might call the operation of *any* lax idempotent monad on a $2$-category a ‘free completion’. See discussion at completion.

Revised on November 6, 2009 01:13:50
by Toby Bartels
(173.60.119.74)