Re: [isabelle] extending well-founded partial orders to total well-founded orders



Hi Larry, 
To prevent Andreas's counterexample, there is no way but to choose the well-ordering in a manner that respects W, namely, as follows: The minimal elements of W, say, forming the set M0, should come first. Then should come the minimal elements of what is left in W after removing M0, say, M1, and so on.  My transfinite construction does just that: it well-orders each M_i and puts the M_i's together as M0 < M1 < M2 < ...
Andrei 

--- On Mon, 2/18/13, Andreas Lochbihler <andreas.lochbihler at inf.ethz.ch> wrote:

From: Andreas Lochbihler <andreas.lochbihler at inf.ethz.ch>
Subject: Re: [isabelle] extending well-founded partial orders to total well-founded orders
To: "Lawrence Paulson" <lp15 at cam.ac.uk>
Cc: "Christian Sternagel" <c.sternagel at gmail.com>, "Andrei Popescu" <uuomul at yahoo.com>, cl-isabelle-users at lists.cam.ac.uk
Date: Monday, February 18, 2013, 4:55 PM

Hi Larry,

As far as I understand your suggestion, it does not even yield a 
ordering. For example, suppose that there are three elements a, b, c 
such that the well ordering <R obtained by the well-ordering theorems 
orders them as a <R b <R c. Now, consider the well-founded relation <W 
which orders c <W a and nothing else. Then, c <TO a as c<W a, and a <TO 
b and b <TO c as neither a <W b, b <W a, b <W c, nor c <W b. Hence,

c <TO a <TO b <TO c

which violates the ordering properties. Or am I misunderstanding something?

Andreas

On 02/18/2013 03:18 PM, Lawrence Paulson wrote:
> I still don't see what's wrong with the following approach:
>
> 1. Prove the well ordering theorem (maybe it has been proved already).
>
> 2. Obtain the desired total ordering as a lexicographic combination of the partial order with the total well ordering of your type
>
> [More specifically: given W a well founded relation and R a well ordering obtained by the well ordering theorem, define TO x y == W x y | (~ W x y & ~W y x & R x y)]
>
> Larry Paulson
>
>
> On 18 Feb 2013, at 06:32, Christian Sternagel <c.sternagel at gmail.com> wrote:
>
>> Dear Andrei,
>>
>> finally deadlines are over for the time being and I found your email again ;)
>>
>> On 01/19/2013 12:22 AM, Andrei Popescu wrote:
>>> My AFP formalization ordinals
>>>
>>> http://afp.sourceforge.net/entries/Ordinals_and_Cardinals.shtml
>>
>> I guess since Isabelle2013 this is now "~~/src/HOL/Cardinals/", right?
>>
>>> (hopefully) provides the necessary ingredients: Initial segments in
>>> Wellorder_Embedding, ordinal sum in theory Constructions_on_Wellorders,
>>> and a transfinite recursion combinator (a small adaptation of the
>>> wellfounded combinator) in theory Wellorder_Relation.
>>
>> Could you elaborate on the mentioned finite recursion combinator and how it is used?
>>
>> thanks in advance,
>>
>> chris
>>
>>
>>
>
>




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