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 August 7th, 2018, 07:06 AM #1 Senior Member   Joined: Nov 2011 Posts: 248 Thanks: 3 zero in the middle of x-axis How I prove that zero is between plus infinity to negative infinity?
 August 7th, 2018, 07:09 AM #2 Senior Member   Joined: Oct 2009 Posts: 733 Thanks: 246 Depends on the specific axioms and definitions you are using. If you list them all, then perhaps we can concoct a proof. Thanks from topsquark
 August 7th, 2018, 07:24 AM #3 Senior Member   Joined: May 2016 From: USA Posts: 1,305 Thanks: 548 Before we try to prove it, perhaps we should determine whether zero is between them or equal to both of them. $\dfrac{1}{-\ \infty} = 0 \implies -\ \infty = 1 * 0 = 0.$ $\dfrac{1}{\infty} = 0 \implies \infty = 1 * 0 = 0.$ $\therefore -\ \infty = 0 = \infty.$ Last edited by JeffM1; August 7th, 2018 at 07:29 AM.
 August 7th, 2018, 09:01 AM #4 Senior Member   Joined: Jun 2015 From: England Posts: 891 Thanks: 269 Because zero is finite.
August 7th, 2018, 09:11 AM   #5
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Quote:
 Originally Posted by studiot Because zero is finite.
Somehow we seem to be getting back to definitions.

August 7th, 2018, 09:29 AM   #6
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Quote:
 Originally Posted by JeffM1 $\dfrac{1}{-\ \infty} = 0 \implies -\ \infty = 1 * 0 = 0.$
Wouldn't $\dfrac{1}{-\ \infty} = 0 \implies -\ \infty = 1 / 0$ make more sense? Why did you put 1 * 0?

August 7th, 2018, 09:32 AM   #7
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Quote:
 Originally Posted by skipjack Wouldn't $\dfrac{1}{-\ \infty} = 0 \implies -\ \infty = 1 / 0$ make more sense? Why did you put 1 * 0?
Yes. I was being silly on purpose. Silly questions warrant silly answers.

 August 7th, 2018, 11:00 AM #8 Senior Member   Joined: Nov 2011 Posts: 248 Thanks: 3 Elaboration text Why the n^2-> infinity greater than the n-> infinity? Can one give me an elaboration text on the idea above. (a link) How many infinities there are? Could it explain the post of my above? Last edited by skipjack; August 7th, 2018 at 06:00 PM.
 August 7th, 2018, 01:06 PM #9 Math Team     Joined: May 2013 From: The Astral plane Posts: 2,040 Thanks: 814 Math Focus: Wibbly wobbly timey-wimey stuff. In your first post you really shouldn't be using infinity as a number. Let's reframe the question as prove that -n < 0 < n when n is very large. The solution I would prefer comes out of Calculus called the intermediate value theorem, but the logic is fairly simple. In order to go from a negative number to a positive number you have to go through 0, don't you? The number system doesn't just "jump" from one real number to another. It is valid to say that $\displaystyle n^2 > n$ for large n. It is not correct to say that $\displaystyle n^2$ represents a "larger infinity" than n. They both represent the same infinity. As to your question: $\displaystyle n^2 > n \implies n^2 - n > 0 \implies n(n - 1) > 0 \implies n > 0 \text{ or } n > 1$. The case of n > 0 is not strictly true because, by inspection, $\displaystyle n^2 < n$ in this domain. Thus we have for n > 1, $\displaystyle n^2 > n$. -Dan Last edited by topsquark; August 7th, 2018 at 01:16 PM.
August 7th, 2018, 01:34 PM   #10
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Quote:
 Originally Posted by shaharhada How I prove that zero is between plus infinity to negative infinity?
You can't and it's not.

Consider a line, infinitely long in each direction. Like Euclid's idea of a line.

Now pick an entirely arbitrary point on the line and call it 0.

Pick another point on the line and call it 1. Now having chosen those two arbitrary points, you can define all the integers, positive and negative whole numbers, based on the unit length between 0 and 1. Then you can define the rationals, all the fractions between the integers. And then you can define the rest of the real numbers by plugging the holes where there are points but not rationals.

So you see that the labelling of the points is completely arbitrary. 0 and 1 could be anything you like.

Here's another way to look at it. You have a bunch of houses on the street. One day the city council decides to renumber all the houses. The addresses of the houses all change. But the houses themselves remain exactly the same as they were before.

A real number is the address of a point on a line. That's all it is. A real number is an address, not the point itself. And addresses are arbitrary, they don't tell you anything about the points themselves.

Likewise in a computer program. The address of a variable is not the same as the contents. The address is just the memory location where the value of some variable lives. The address is not the variable. It's only the address.

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