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Topic: PREDICATE CALCULUS 2
Replies: 23   Last Post: Nov 29, 2012 1:13 AM

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Virgil

Posts: 7,011
Registered: 1/6/11
Re: PREDICATE CALCULUS 2
Posted: Nov 28, 2012 3:20 PM
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In article
<d5c8fa17-85ea-4bca-9491-420151cc2344@me7g2000pbb.googlegroups.com>,
Graham Cooper <grahamcooper7@gmail.com> wrote:

> On Nov 29, 1:13 am, Dan Christensen <Dan_Christen...@sympatico.ca>
> wrote:

> > On Nov 28, 1:35 am, Graham Cooper <grahamcoop...@gmail.com> wrote:
> >
> >
> >
> >
> >
> >
> >
> >
> >

> > > On Nov 28, 2:26 pm, Dan Christensen <Dan_Christen...@sympatico.ca>
> > > wrote:

> > > > On Nov 27, 8:59 pm, Graham Cooper <grahamcoop...@gmail.com> wrote:
> >
> > > > > There are 2 ALLs which is more complicated but you can format it as a
> > > > > SUBSET using a cartesian product of the 2 X values with a common Y.

> >
> > > > > isfunction(r)  <-  ALL(Y1) ALL(Y2) r(X,Y1)^r(X,Y2) -> Y1=Y2
> >
> > > > There is more to functionality than this. I may not fully understand
> > > > your unusual notation (PROLOG?), but it would seem you have left out
> > > > the requirement that FOR ALL elements of some domain set, THERE EXISTS
> > > > a unique image in a codomain set. (This is where I think quantifiers
> > > > become indispensable).

> >
> > > > > {(Y1,Y2)|r(X,Y1)^r(X,Y2)} C {(Y1,Y2)|r(X,Y1)^r(X,Y2)->Y1=Y2}
> >
> > > > [snip]
> >
> > > > The same comment applies... I think.
> >
> > > > Anyway, I am still waiting for proofs of the following:
> >
> > > > 1. {(x,y) | x in S, y=x}  is a function mapping the set S onto itself
> > > > 2. {x | ~x in x} cannot exist
> > > > 3. {x | x=x} cannot exist

> >
> > > > You really need to address these fundamental results.
> >
> > > > For what it is worth, and from what little I know about PROLOG, it
> > > > doesn't seem to be capable of all that is required to do mathematical
> > > > proofs in general. It may be able to model some interesting and useful
> > > > aspects of predicate logic and set theory, but, for your purposes,
> > > > important pieces of the puzzle seem to be missing.

> >
> > > Nope, this is exactly the definition of function.
> >
> > > {(Y1,Y2) | r(X,Y1)^r(X,Y2)}  C  {(Y,Y) | r(X,Y)}
> >
> > > which simply guarantees only 1 Y value for any X value.
> >
> > This is a common mistake. According to this erroneous view, every set
> > {(x,y)} is a function for any objects x and y. The functionality of a
> > set of ordered pairs is always defined in terms of a domain and
> > codomain set. Here is a typical formal(ish) definition of a function
> > from Wiki (my comments in []'s):
> >
> > "A function f from X [the domain of f] to Y [the codomain of f] is a
> > subset of the Cartesian product X × Y subject to the following
> > condition: every element of X is the first component of one and only
> > one ordered pair in the subset.[3] In other words, for every x in X
> > there is exactly one element y such that the ordered pair (x, y) is
> > contained in the subset defining the function
> > f."http://en.wikipedia.org/wiki/Function_(mathematics)#Definition
> >
> > Example: Let X=Y={0,1}.
> >
> > Then {(0,0), (1,1)} is function from X to Y, while {(0,1)} is not.
> >

>
>
> Semantics escapes you!
>
> {(0,1)} is a function.


But not a function FROM X = {0,1} to anything. At least according to the
definition above.
>
> You can return the domain of r(X,Y)
> with another predicate.
>
> Herc

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