[Flirts] Re: Power institution comorphisms

[Till Mossakowski]

Dear Carlos,

>> See SalibraScollo92,SalibraScollo96, Scollo93 in the bibliography
>> on morphisms:
>> http://www.informatik.uni-bremen.de/flirts/Morphisms.bib
>> Also, Tarlecki87 should contain some similar notion.
> 
> Ok, perfect. I just wonder if I can find these oldies.

Just ask the authors. If you don't get the papers from them, I can
send you copies. Unfortunately, I only have paper copies.
It would be nice to have a FLIRTS bibliography with online papers
also for the older publications.

>> I found that in most examples, you can just use a forward comorphism
>> instead, that is, you can map models to models in the opposite
>> direction. (Maps into the power set are just relations, and often
>> the relation, if not functional, is inverse-functional.)
>>
> Indeed. That is also the case in our examples. The problem is that it is 
> relatively straightforward to come up with a counterpart of surjectivity 
> that may allow sound and complete borrowing for the case of these power 
> model comorphisms, but it seems to be hopeless in general for forward 
> comorphisms. And we want these maps to be witnesses of the fact that we 
> are in the presence of a conservative extension.

Yes, for borrowing along forward (co)morphisms, you need
surjectivity of the model translation, as usual. With the use of power
(co)morphisms, this can be relaxed somewhat.

Greetings,
Till

-- 
Till Mossakowski               Phone +49-421-218-4683
Dept. of Computer Science      Fax +49-421-218-3054
University of Bremen           till at tzi.de
P.O.Box 330440, D-28334 Bremen http://www.tzi.de/~till