Re: discrete probability functor on Rel

[Paul B Levy <P.B.Levy@cs.bham.ac.uk>]

Dear Nathan,

Your suggestion of deducing the countable splitting lemma from the
finite one using the finite intersection property doesn't work, because
the set of distributions on Nat is not compact.  For example, let d_n be
the distribution that is 1 on n and 0 everywhere else.  This converges
pointwise to the "distribution" that is 0 everywhere.

So I still don't know how to prove the countable splitting lemma.  Maybe
you can get a student to write up Nawrotsky's algorithm in English...

Paul


On 27/07/16 12:27, Paul B Levy wrote:
> Dear all,
>
> I learnt years ago that the following definition gives an endofunctor D
> on the category Rel of sets and relations.  But I have never seen a proof.
>
> - For a set A, let DA be the set of discrete probability distributions
> on A, i.e. functions from A to nonnegative reals whose sum is 1.  (Such
> functions are necessarily countably supported.)
>
> - For a relation R : A -> B, let DR : DA -> DB relate d in DA to d' in
> DB when for every subset U of A, we have d(U) <= d'(R[U]), writing R[U]
> for the set of all R-images of elements of U.  (It follows that for
> every subset V of B, we have d'(V) <= d(R^c[V]), writing R^c for the
> converse of R.)
>
> To show D is a functor, the hard part is to show that, for relations R :
> A --> B and S : B --> C, and d in DA and d'' in DC with (d,d'') in
> D(S.R), there exists d' in DB such that (d,d') in DR and (d',d'') in DS.
> I would like to see a proof of this.
>
> It's the discrete case of Theorem 1 (i)=>(ii) in
>
> https://projecteuclid.org/euclid.aop/1176995659
>
> where two proofs are mentioned.  Firstly, Nawrotski's, which is in
> German, which I can't read:
>
> Nawrotski K (1962) Eine Montonieeigenschaft zufaelliger Punktfolgen,
> Math. Nachr. 24, pages 193-200.
>
> Secondly, Strassen's:
>
> https://projecteuclid.org/euclid.aoms/1177700153
>
> which is in English but beyond my comprehension.  Strassen also cites
> papers of Kellerer (in German) that might also contain a proof.
>
> Can anyone give me a proof, please?
>
> Two other things:
>
> - For the subfunctor D_f of finitely supported distributions, the result
> is easier.  It's the "splitting lemma" of probabilistic powerdomain
> theory, and was proved by Jones, and later by Jung and Tix, and earlier
> by Berge and dall'Aglio (cited by Strassen).  But these proofs (at
> least, the ones I've seen) don't seem to adapt to the countably
> supported case.
>
> - I know that D, being an endofunctor on Set that preserves weak
> pullbacks, has a unique extension to an endofunctor on Rel.  But that
> doesn't help.
>
> Regards,
> Paul
>
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-- 
Paul Blain Levy
School of Computer Science, University of Birmingham
http://www.cs.bham.ac.uk/~pbl


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