# Apostol: infinity as finite point

1. Nov 4, 2011

### kahwawashay1

I found a torrent online of Apostol's "Mathematical Analysis" 1st edition and I think I found a typo, or whoever scanned the book cut off the edge a bit...

Apostol writes that the extended real number system R* is denoted by [-∞, +∞] while the regular real number system R is denoted by (-∞, ∞). Then he says "The points in R are called 'finite' to distinguish them from the 'infinite' points +∞ and -∞. " Shouldn't it read that the points in R* are finite, not the points in R? Because if you include infinity in the interval, you are treating it as a finite point?

The "R" is the sentence that I quoted above is a the very edge of the page so it could be that the scanner of the book cut off the " * " accidentally, but I just want to confirm...

2. Nov 4, 2011

### SteveL27

I think he means that the points in R (as a subset of R*) are called finite. So in R*, the numbers 3, -6, and pi are finite. +/-infinity are infinite.

3. Nov 4, 2011

### kahwawashay1

ohhh i see...so basically R, by not including infinity, contains only finite points, and the opposite is true of R*. thanks Steve!

4. Nov 4, 2011

### AlephZero

"Infinty" is not included in the open interval $(-\infty,\infty)$. An open interval does not contain its end points.

Be careful about the notation for an open inteval ( ) and a closed interval [ ].

Last edited: Nov 4, 2011
5. Nov 4, 2011

### kahwawashay1

thanks but I do know the difference between open and closed interval.
R* is denoted by closed interval [-∞,∞] and R by open. I am assuming you thought that there cannot be a thing as [-∞, ∞] ? Thats what I thought too until I read the above mentioned book lol

6. Nov 8, 2011

### remoa

can you tell me where did you find the torrent for the first edition cause i can only found the second one and there is some proof wich is only in the first one
Thank you

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