# Re: compound propositions

Date: Thu, 18 Mar 2010 14:09:05 -0300

Message-ID: <4ba25dbd$0$12465$9a566e8b_at_news.aliant.net>

> David BL wrote:

*> ...
**>
*

>> So back to your statement, I would say that there is a 1:1 >> correspondence from relation (= set of tuples) to a boolean valued >> function that is true for tuples in that relation and false >> otherwise.

Indeed. One is the predicate and the other is the extension of the predicate. A relation is an extension of a predicate.

>> This boolean valued function can be said to represent a >> predicate under an interpretation but I'm not sure if that's what you >> mean. More specifically, what do you mean by "satisfy" when you say >> relations satisfy predicates? >> ...

*>*

> A relation satisfies one or more predicates when its attribute names

> match the variable names apparent in the predicates and the attribute

*> types are applicable for whatever manipulations (eg., aggregation) the
**> predicate states.
*

In other words, the extension of a predicate is the set of all tuples that satisfy the predicate.

> (I just say 'satisfies' to avoid suggesting that a predicate has a truth

*> value. Don't see anything wrong with talking about correspondence
**> between relations and functions, just seems more to the point to think
**> of another correspondence - tuples that indicate whether propositions
**> are true or false.)
*

Received on Thu Mar 18 2010 - 18:09:05 CET