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## #1 2014-04-29 12:19:15

Reuel
Member
Registered: 2010-11-28
Posts: 178

### Impredicativity

Hello. I have a definition question.

According to wikipedia and wolfram alpha, impredicativity is the self-reference of a set. What would be a simple example of this?

If we have a point on (x,f(x)), say, (a,b) and separate the two points as

and

so that either self-referencing result is not a function but is, at the specified value, the value itself... would this be a good example of the concept? Each result has x or f on both sides of the equals sign so that if x=a and f=b then we revert to our original values that constitute a single point.

Thanks.

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## #2 2014-04-29 20:20:12

bob bundy
Registered: 2010-06-20
Posts: 8,065

### Re: Impredicativity

hi Reuel,

Here's an example:

I have a large library of books.  Some of them have red covers.  I have decided to make a catalogue of all my red books, called the Red Books Catalogue.  I have decided to make this as a book, and give it a red cover.  Thus, it is listed in the catalogue, as a book with a red cover.

Bob

ps.  Consider the other catalogue I have made of all the books which don't contain themselves as an entry.  I've made the catalogue but I cannot decide whether to list the catalogue as an entry.

Children are not defined by school ...........The Fonz
You cannot teach a man anything;  you can only help him find it within himself..........Galileo Galilei

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## #3 2014-05-05 08:07:45

Reuel
Member
Registered: 2010-11-28
Posts: 178

### Re: Impredicativity

Hey bob.

Do recursive statements such as the self-referencing I gave in my example have any special name, or are they just called recursive statements?

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## #4 2014-05-05 19:28:39

bob bundy
Registered: 2010-06-20
Posts: 8,065

### Re: Impredicativity

hi Reuel,

I wouldn't have called them by that name but I cannot see what is wrong with it.  I don't know any better name.  I would probably just have said something like: "So x is defined in a function that contains x itself." which doesn't exactly 'trip off the tongue' easily.

On-line I found these pages, which might be worth a look:

http://www.mathwords.com/r/recursive_formula.htm

http://www.wolframalpha.com/input/?i=re … lk=4&num=3

Bob

Children are not defined by school ...........The Fonz
You cannot teach a man anything;  you can only help him find it within himself..........Galileo Galilei

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