MathOverflow is a question and answer site for professional mathematicians. Join them; it only takes a minute:

Sign up
Here's how it works:
  1. Anybody can ask a question
  2. Anybody can answer
  3. The best answers are voted up and rise to the top

Instead of taking a one to one correspondence meaning each set has the same number of elements. why not use the concept of coverings of topology? The irrational numbers covers the whole numbers but not vice versa?

A hierarchy of coverings instead of infinities. Wouldn't that make those infinities more manageable in those terms?( yes I know topology can be expressed in set theory)

share|cite|improve this question

closed as not a real question by Reid Barton, Tom Leinster, Ben Webster Mar 25 '10 at 14:54

It's difficult to tell what is being asked here. This question is ambiguous, vague, incomplete, overly broad, or rhetorical and cannot be reasonably answered in its current form. For help clarifying this question so that it can be reopened, visit the help center.If this question can be reworded to fit the rules in the help center, please edit the question.

This is in danger of being closed as "not a real question". As the answers below testify, at present it is simply unclear what you mean. Please clarify... – Pete L. Clark Mar 25 '10 at 14:54
In particular, why do you want to "dispense with the notion of infinity"? What is unmanageable about the current notions of infinity? – Pete L. Clark Mar 25 '10 at 14:56

By $X$ covers $Y$ I assume you mean there exists a surjection $f:X \to Y$. The theory you're describing is exactly the same as the standard theory of cardinal numbers. In fact, if $X$ "covers" $Y$ and $Y$ "covers" $X$ then there is a bijection between $X$ and $Y$ . The proof is pretty and easy and is a good homework problem. You could also look it up in the beginning of any book that introduces the cardinals. Aside: I don't see what this has to do with topology. I also don't understand what you mean by "Dispensing with the notion of infinity..."

share|cite|improve this answer

I don't quite know what you mean by "coverings of topology", but it is possible to formalize a notion of size for infinite sets which relies on the part-whole conception, rather than the bijective correspondence conception. These two views are mutually exclusive, in the sense that size for finite sets satisfies both properties, infinite sets can only support one of the two. But either choice can be made to work!

So, if you require the notion of size to equate two bijective sets, then the even numbers are equal in size to the natural numbers (this is the traditional Cantorian view). You could also take a mereological view, and say that one set is smaller than another if every element of the first set is a member of the second set. In this interpretation, the even numbers are smaller than the natural numbers.

A recent issue of the Review of Symbolic Logic had an article about these issues, including both some history of mathematics and more recent logical systems which formalize the mereological view. See Paolo Mancosu's "Measuring the Size of Infinite Collections of Natural Numbers: Was Cantor's Theory of Infinite Number Inevitable?"

share|cite|improve this answer

Not the answer you're looking for? Browse other questions tagged or ask your own question.