Let $F : C \to D$ be a functor between categories with filtered colimits, and let Cat be the category of categories. Let $(M, μ, η)$ be a monad on Cat. What properties of $M$ are necessary to ensure that
[p] F preserves filtered colimits if and only if M F has a right adjoint.
Here's a slightly different situation that I'm interested in:
Let
- $\text{Poset}$ be the category of partially ordered sets (posets).
- $\text{CompPoset}$ be the category of complete partial orders with join preserving poset maps as maps
- $\text{Meet}$ be the category of partial orders with meets, and with meet preserving poset maps as maps.
- $\text{Frame}$ be the category of frames. Maps of frames must preserve meets and joins.
- use op for the opposite poset, opposite complete poset, opposite map, etc.
There are adjunctions
- $L : \text{Poset} \leftrightarrow \text{Meet} : R$ ($L$ freely adds in meets and $R$ forgets)
- $CL : \text{CompPoset} \leftrightarrow \text{Frame} : CR$ ($CL$ freely adds in meets among complete partial orders and $CR$ forgets).
I am trying to prove or disprove:
- For a complete poset $X$, $L X = CL X$. That is, $L X$ has the universal property of $CL X$.
- For $f : X \rightarrow Y$ a map of partially ordered sets with filtered colimits, $L op f$ has a right adjoint if and only if $f$ preserves filtered colimits.
- For $f: X \rightarrow Y$ a map of complete partially ordered sets, $f$ preserves filtered colimits if and only if $CL f$ has a right adjoint.
2 is the more important of these for me. And note that 3 follows from 1 and 2.