functor of descent type

A functor is of *descent type* if it satisfies “half” of the condition to be monadic.

A functor $U:B\to A$ is of **descent type** (or is **premonadic**) if

- it has a left adjoint $F:A\to B$, and
- for all $b\in B$ the canonical fork $F U F U b \underoverset{F U \epsilon}{\epsilon F U}{\rightrightarrows} F U b \xrightarrow{\epsilon} b$ is a coequalizer.

The second condition is equivalent to asking that the comparison functor $B \to A^{U F}$ from $B$ to the Eilenberg-Moore category of the monad $U F$ is fully faithful. When the comparison functor is essentially surjective (hence an equivalence of categories), $U$ is of effective descent type or monadic.

By monadic descent, a morphism $f$ in the base of a fibration is a descent morphism if and only if $f^*$ is of descent type. This is presumably the origin of the terminology; $f$ is an effective descent morphism when $f^*$ is monadic.

Last revised on July 11, 2021 at 10:15:24. See the history of this page for a list of all contributions to it.