# R set magnitude

by Lama
Tags: magnitude
 P: 467 "real line" is used to mean real axis, i.e., a line with a fixed scale so that every real number corresponds to a unique point on the line. http://mathworld.wolfram.com/RealLine.html If the real-line scale is fixed, then how a part of the real-line has the same magnitude of the real-line?
 PF Patron Sci Advisor Emeritus P: 11,137 Two sets have the same cardinality if you can find a one-to-one correspondence that maps one set onto the other.
 PF Patron Sci Advisor Emeritus P: 16,094 Consider this an advance warning, Organic; if you try to use this thread for theory development, it will be deleted. You've used up all of your second chances.
P: 467

## R set magnitude

Ok Hurkyl,

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 Quote by Hurkyl Consider this an advance warning, Organic; if you try to use this thread for theory development, it will be deleted. You've used up all of your second chances.
Organic ? Who's Organic ?
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 Quote by Lama Ok Hurkyl, I got it, can you please answer to my question?
Cantor proved this - try Googling it. In fact $$R^n$$ - n-dimensional real space - is as big too.
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 If the real-line scale is fixed, then how a part of the real-line has the same magnitude of the real-line?
Well, the main question is what do you mean by "part" and what do you mean by "magnitude"?

I will assume by "part" you mean an interval whose endpoints are not the same.

If by "magnitude" you mean simply the cardinality of the sets, then it's done by showing a 1-1 correspondence. For example, a 1-1 correspondence between the interval (0, 1) and the whole real line can be given by the function f(x) = (1-2x)/(x (1-x)); this function maps from the interval (0, 1) onto the entire real line.

If by "magnitude" you mean length, then they generally don't have the same magnitude; the real line has infinite length, while most intervals don't.
 P: 467 Hurkyl, I am talking about a bijection between the real-line and any arbitrary part (or a subset if you will) of it. So, If the real-line scale is fixed, then how a part of the real-line has the same magnitude of the real-line?
 PF Patron HW Helper Sci Advisor Thanks P: 11,935 What is a "fixed real line scale", and whatever has that to do with the existence of bijections
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Well, the choice of scale is irrelevant to cardinailty; the line consists of the exact same points no matter which ones I choose to call "0" and "1". So, the question reduces to:

 how a part of the real-line has the same magnitude of the real-line?

I gave an example of how the interval (0, 1) has the same cardinality as the real line, by presenting the bijection f(x) = (1 - 2x) / (x (1 - x)).

Here is an example of how a part might not have the same cardinality as the real line: the set of points {0, 1, 2} has finite cardinality, but the real line does not have finite cardinality.
P: 467
 the choice of scale is irrelevant to cardinailty
I am sorry, but please look at http://mathworld.wolfram.com/RealLine.html

As you see, the words "fixed scale" are used.
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 Quote by Lama I am sorry, but please look at http://mathworld.wolfram.com/RealLine.html As you see, the words "fixed scale" are used.
He didn't say you should use the words, he said they were not irrelevant to cardinality.
 HW Helper Sci Advisor P: 9,395 Organic, www, lama, dialog, whatever today, these things are all definitions. Why do you not accept that? Fixed scale is not a well defined term, and I would hesitate to say what it means, but presumably it is to do with distances being euclidean. That has nothing to do with cardinality. "size" is a vague word, and just as we would never say "amount" when referring to chemistry if we were being precise, you need to distinguish between the suggestive descriptions offered as an aid to understanding and the formal rigorous definitions that are used when we come to practise mathematics.
 P: 27 I have found this thread http://www.physicsforums.com/showthr...438#post228438 .
 HW Helper Sci Advisor P: 9,395 Please can someone lock this? Don't suppose he's on a static IP is he?
 P: 27 Can you explain f(x) = (1 - 2x) / (x (1 - x))?
 PF Patron Sci Advisor Thanks Emeritus P: 38,395 What do you mean "explain" it? It's a rational function defined for all real numbers except and 1. It has vertical asymptotes at x= 0 and x= 1 and a horizontal asymptote y= 0. What more do you want to know about it?
 P: 27 This function maps from the interval (0, 1) onto the entire real line. But it is not explain how it can be done if the real line has no fractal structure (self similarity over scales). So, where can I find a mathematical theory that explains why it can be done?

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