Try   HackMD

Monotone Predicate Liftings

(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

U and the downset functor
D
.

Set Functors

We have studied modal operators as predicate liftings

P22

where

P2 can be pictured as [1]

Image Not Showing Possible Reasons
  • The image file may be corrupted
  • The server hosting the image is unavailable
  • The image path is incorrect
  • The image format is not supported
Learn More →

Which order on

P2 exhibits the monotone modal operators as exactly the monotone predicate liftings?

The answer is given by the so-called posetification [2]

P:PosPos

of the functor

P:SetSet.

Fact:

P is the convex powerset functor, that is, for two convex subsets
a,bX
we have
ab  iff xa.yb.xy and yb.xa.xy

Exercise: Recall that

2={0<1}. The order on
P2
is given by
{0}<{0,1}<{1}
:

Image Not Showing Possible Reasons
  • The image file may be corrupted
  • The server hosting the image is unavailable
  • The image path is incorrect
  • The image format is not supported
Learn More →

It follows that there are

2×4 monotone predicate liftings
P22
. The 4 upsets of the chain are
{0,01,1},{01,1},{1},{}

We can now list all monotone predicate liftings as follows.

subset of
P2
Boolean combination of generators modal formula
{}
{1}
¬oi
ϕϕ
{01,1}
i
ϕ
{0,01,1}
oi
{}
¬o¬i
{,1}
¬o
ϕ
{,01,1}
¬oi
ϕϕ
{,0,01,1}

Summary

We have seen that in the example of the powerset functor that the monotone modal operators correspond precisely to the monotone functions

P22 where
P
is the convex powerset functor.

More generally, for any set-functor

T, the
n
-ary monotone modal operators (=predicate liftings) are in bijection to the monotone functions
T(2n)2

where

T is the posetification of
T
.

Poset Functors

The Upset Functor

Let

U be the upset functor. Consider
UXUϕU22

U2 is the poset (again
01={0,1}
, etc)
01<1<

which allows us to identify monotone functions

U22 with upsets:

upset of
U2
modal formula
{01,1,}
{1,}
ϕ
{}
{}

The difference between upsets of

P2 and upsets of
U2
is that in the latter case the "observation"
{01,1}=(oi)(¬oi)=i which corresponds to ϕ

is not an upset of

U2.

The Downset Functor

Let

D be the downset functor. Consider
DXDϕD22

D2 is the poset
<0<01

which allows us to identify monotone functions

D22 with upsets:

upset of
D2
modal formula
{,0,01}
{0,01}
{01}
ϕ
{}

The difference between upsets of

P2 and upsets of
D2
is that in the latter case
{,1}=(¬o¬i)(¬oi)=¬o which corresponds to ϕ

is not available since

1 is not an element of
D2
.

Breakdown of Duality

It is interesting to think about why the arguments for

U2 and
D2
are not entirely dual.

On the "object level" there is duality: Swapping

0 and
1
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

U and
D
, the meaning of the predicate liftings depends on the existential quantifier implicit in taking the direct image of
ϕ
. For example, the statement
ϕ[S]=1 only if ϕ[S]01

is on the meta level and does not dualise. [3]

We summarise the situation with the following picture, depicting

P2,
U2
,
D2
and the subsets that correspond to
and
:

Image Not Showing Possible Reasons
  • The image file may be corrupted
  • The server hosting the image is unavailable
  • The image path is incorrect
  • The image format is not supported
Learn More →

Distribution Functor

Proposition: The posetification of the distribution functor is given by

pq if
p[S]q[S]
for all upsets
S
.

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).

References

The definition of the posetification (extension from

Set to
Pos
) 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

T are the monotone predicate liftings of
T
.


  1. Recall that for a predicate lifting

    and a formula
    φ:X2
    , the meaning of
    ϕ
    is given by how the composition
    PXPϕP22
    acts on a subset 'of successors'
    SPX
    . ↩︎

  2. The posetification of a functor

    T is defined as follows. Any poset
    (X,R)
    can be represented by its "graph"
    p1,p2:RX

    Moreover, one can reconstruct the poset

    (X,R) as a certain order-enriched colimit of the diagram
    p1,p2:RX
    . Then the posetification
    TX
    is defined as the said colimit of
    Tp1,Tp2:TRTX
    .

    The posetification enjoys the universal property of an order-enriched left Kan extension. ↩︎

  3. "

    ϕ[S]=1 only if
    ϕ[S]01
    " is needed to establish that
    {01}D2
    corresponds to
    ϕ
    . ↩︎