(draft …up … predicate liftings … expressive predicate liftings)
In a previous note, we defined predicate liftings and showed that the unary predicate liftings of the powerset functor are isomorphic to the free Boolean algebra generated by and .
Here, we first single out all of those 16 modal operators that are monotone.
Then we are going to test our insights by studying the predicate liftings of two posets functors, namely the upset functor and the downset functor .
We have studied modal operators as predicate liftings
where can be pictured as [1]
Which order on exhibits the monotone modal operators as exactly the monotone predicate liftings?
The answer is given by the so-called posetification [2]
of the functor .
Fact: is the convex powerset functor, that is, for two convex subsets we have
Exercise: Recall that . The order on is given by :
It follows that there are monotone predicate liftings . The 4 upsets of the chain are
We can now list all monotone predicate liftings as follows.
subset of | Boolean combination of generators | modal formula |
---|---|---|
We have seen that in the example of the powerset functor that the monotone modal operators correspond precisely to the monotone functions where is the convex powerset functor.
More generally, for any set-functor , the -ary monotone modal operators (=predicate liftings) are in bijection to the monotone functions
where is the posetification of .
Let be the upset functor. Consider
is the poset (again , etc)
which allows us to identify monotone functions with upsets:
upset of | modal formula |
---|---|
The difference between upsets of and upsets of is that in the latter case the "observation"
is not an upset of .
Let be the downset functor. Consider
is the poset
which allows us to identify monotone functions with upsets:
upset of | modal formula |
---|---|
The difference between upsets of and upsets of is that in the latter case
is not available since is not an element of .
It is interesting to think about why the arguments for and are not entirely dual.
On the "object level" there is duality: Swapping and and turning the order around is an isomorphism.
This means that it must be the "meta level" which is breaking the duality.
Importantly, for both and , the meaning of the predicate liftings depends on the existential quantifier implicit in taking the direct image of . For example, the statement
is on the meta level and does not dualise. [3]
We summarise the situation with the following picture, depicting , , and the subsets that correspond to and :
Proposition: The posetification of the distribution functor is given by if for all upsets .
This order on distributions goes back to (Jones and Plotkin, 1989) for so-called IPOs and was extended to all posets in Worrell 2000. The observation that it is the posetification will appear in (Kurz and Motamed, 2022).
The definition of the posetification (extension from to ) of an arbitrary set-functor as well as the result on monotone predicate liftings is in
Theorem 7.1 contains the result that the predicate liftings of the posetification of are the monotone predicate liftings of .
Recall that for a predicate lifting and a formula , the meaning of is given by how the composition acts on a subset 'of successors' . ↩︎
The posetification of a functor is defined as follows. Any poset can be represented by its "graph"
Moreover, one can reconstruct the poset as a certain order-enriched colimit of the diagram . Then the posetification is defined as the said colimit of .
The posetification enjoys the universal property of an order-enriched left Kan extension. ↩︎
" only if " is needed to establish that corresponds to . ↩︎