# Dividing by infinity, exactly, finally!

• B
• jaketodd
In summary, the extended real number line and projectively extended real line are alternative number systems that allow for division by infinity to result in an exact value of zero. However, these systems do not satisfy the field axioms and can cause more problems than they solve. Additionally, the idea of dividing by infinity is not always well-defined and can lead to different values depending on the growth rates of the numerator and denominator. Therefore, these systems are not commonly used in mathematics and should be approached with caution.
TeethWhitener said:
Ok so then either ##2\times\infty## is infinity or it is a finite real number. So @jbriggs444 ‘s point is still valid.
Is 2x5 the same as 2x10? Infinity can be defined, so no.

PeroK
jaketodd said:
Is 2x5 the same as 2x10? Infinity can be defined, so no
But can it be defined with the properties that you wish?

That the non-standard reals contain at least one infinite element is well accepted. However, as I understand it, there is no definition available within the non-standard reals that can distinguish between "finite" and "infinite". That is, the set of infinite reals is an external set, not an internal set. So you are going to have a hard time constructing ##\infty## or usefully defining it.

Note that the non-standard reals are something completely different from the two point compactification of the real line (with exactly two infinities) or the one point compactification of the complex plane (with exactly one infinity) that were mentioned in the OP.

Last edited:
PeroK
jaketodd said:
I think we need to study the alternative number systems more. I just know that infinity is defined in both I mention, instead of being undefined and just an idea. You say "finite real number." Both of the alternative number systems I mention are different than the real number system, as mentioned in those two Wikipedia articles. Related, but different.
You are correct. There are alternative number systems with definitions of ##\infty## that have been studied very thoroughly. They may not be as simple as you indicated in your original post, but they are very interesting in certain applications.

jaketodd said:
Is 2x5 the same as 2x10? Infinity can be defined, so no.
This is a non sequitur. ##2\times5\neq2\times10## regardless of whether infinity is defined.

Here’s the deal. You need to define what ##2\times\infty## means. If it’s a number in the extended real number system or the projective system, then it’s either equal to ##\infty##, ##-\infty##, or a real number. Otherwise you have to add ##2\times\infty## separately to the system with its own arithmetic rules. That’s fine. People have done that; they’re called transfinite ordinal numbers, and there’s a rich theory surrounding them (including an internally consistent arithmetic). The main piece of the puzzle is how an element like ##2\times\infty## fits into the order of the system. Is it greater, less than, or neither in relation to the other numbers? On the extended real line, all the numbers can be ordered from least to greatest, and every subset can as well. In the projective real line, there is no good way to order numbers, since infinity is simultaneously greater than and less than all the numbers. So in a real sense, we can’t say that ##2<1## is false in this system. This is exactly analogous to saying we can’t tell whether one point on a circle is greater or less than another point.

The point of all this is to say that every system is going to have its trade offs. As @pbuk pointed out, these systems don’t satisfy the field axioms. In particular, the infinity elements do not have a multiplicative or additive inverse.

FactChecker and PeroK
jaketodd said:
I think we need to study the alternative number systems more.
I think you need to study these systems more; they have already been studied extensively by mathematicians and their properties and limitations are well known as you should be able to see from the depth and consistency of the replies in this thread.

FactChecker and PeroK
pbuk said:
I think you need to study these systems more; they have already been studied extensively by mathematicians and their properties and limitations are well known as you should be able to see from the depth and consistency of the replies in this thread.
And on this note, time to close the thread. Thanks to all that have participated.

FactChecker, Mark44, topsquark and 1 other person

• General Math
Replies
36
Views
4K
• Topology and Analysis
Replies
2
Views
487
• General Math
Replies
63
Views
4K
• General Math
Replies
35
Views
11K
• General Math
Replies
12
Views
3K
• Atomic and Condensed Matter
Replies
1
Views
1K
• Optics
Replies
5
Views
1K
• General Math
Replies
8
Views
3K
• Calculus
Replies
11
Views
401
• General Math
Replies
3
Views
1K