# What is meant by ∞ ?

1. Oct 27, 2013

### jacket

I know ∞ is not really a number. It represents 'greater than every real number'. And for any real number x, we can say
-∞ < x < +∞

Now, my questions are -
(A) how come arithmetic operators interact with ∞ if it is not a number?
(B) what are the results for the expressions below?
(C) and also why we do have results for these if ∞ is not a number?

01. (+∞) + (+∞) =
02. (-∞) + (-∞) =
03. (+∞) + (-∞) =
04. (+∞) - (+∞) =
05. (+∞) - (-∞) =
06. (-∞) - (-∞) =
07. (+∞) * (+∞) =
08. (-∞) * (+∞) =
09. (-∞) * (-∞) =
10. (+∞) / (+∞) =
11. (+∞) / (-∞) =
12. (-∞) / (+∞) =
13. (-∞) / (-∞) =
14. (+∞) + any real number > 0 =
15. (-∞) + any real number > 0 =
16. (+∞) - any real number < 0 =
17. (-∞) - any real number < 0 =
18. (+∞) * (any real number > 0) =
19. (+∞) * (any real number < 0) =
20. (-∞) * (any real number > 0) =
21. (-∞) * (any real number < 0) =
22. (+∞) * 0 =
23. (-∞) * 0 =
24. (+∞) / (any real number > 0) =
25. (+∞) / (any real number < 0) =
26. (-∞) / (any real number > 0) =
27. (-∞) / (any real number < 0) =
28. (+∞) / 0 =
29. (-∞) / 0 =
30. (+∞) ^ (any real positive number except 0, 1) =
31. (+∞) ^ (any real negative number except 0, -1) =
32. (-∞) ^ (any real positive number except 0, 1) =
33. (-∞) ^ (any real negative number except 0, -1) =
34. (+∞) ^ 0 =
35. (+∞) ^ 1 =
36. (+∞) ^ -1 =
37. (-∞) ^ 0 =
38. (-∞) ^ 1 =
39. (-∞) ^ -1 =
40. (+∞) ^ (+∞) =
41. (-∞) ^ (-∞) =
42. (+∞) ^ (-∞) =
43. (-∞) ^ (+∞) =
44. 1 / (+∞) =
45. 0 / (+∞) =
46. 1 / (-∞) =
47. 0 / (-∞) =
48. 0 / 0 =
49. 1 / 0 =
50. 0 / (+∞) =
51. 0 / (-∞) =
52. 1 ^ (+∞) =
53. 1 ^ (-∞) =
54. 0 ^ (+∞) =
55. 0 ^ (-∞) =

I know that is long. But I will very much appreciate your help.

2. Oct 27, 2013

### voko

Every expression in the list above should be interpreted as a shortened form of a problem involving limits. For example, #1 stands for $$\lim_{x, \ y \ \to +\infty} (x + y) = +\infty$$ This can be proved rather trivially. $x \to \infty$ means that for any given $X > 0$ there is $x > X$; same for $y$. Thus, if given any $Z > 0$, let $X = Y = Z/2$, then there are $x > X = Z/2$ and $y > Y = Z/2$, so $x + y > Z$, which means $(x + y) \to \infty$.

#22 is trickier. It can mean two things: $$\lim_{x \to +\infty} x \cdot 0$$ and $$\lim_{x \to +\infty, \ y \to 0 } x \cdot y$$ The first of these is zero. The second cannot be resolved unless some relationship between $x$ and $y$ is known. For example, if $y = x^{-1}$, then, obviously, the limit is 1. If $y = x^{-2}$, then the limit is zero. If $y = x^{-1/2}$, the limit is $+\infty$. If $y = -x^{-1/2}$, the limit is $-\infty$.

3. Oct 27, 2013

### arildno

1. "I know ∞ is not really a number"
----------------
Incorrect.
Infinity is not a real number, but might perfectly well be a number in another number system than the reals.

4. Oct 27, 2013

### goldust

5. Oct 27, 2013

### arildno

Am I allowed to prefer it to be equal to 5?

6. Oct 27, 2013

### Number Nine

You're equivocating here. The "infinity" referred to in set theory (i.e. the cardinal numbers) is not the same as the "infinity" referred to in analysis in the context of limits, or the extended real line. The word has many different meanings in different disciplines.

7. Oct 29, 2013

### HallsofIvy

Staff Emeritus
Of course- for sufficiently large values of 5!

8. Oct 30, 2013

### arildno

9. Oct 30, 2013

### skiller

Why bring 120 into this?

10. Oct 30, 2013

### Staff: Mentor

The exclamation point is punctuation, not factorial. HoI is being facetious...

11. Oct 30, 2013

### skiller

You don't say...

12. Oct 30, 2013

### Staff: Mentor

I couldn't tell whether you were asking seriously or were attempting to be humorous...

13. Oct 30, 2013

### skiller

If you seriously thought I was talking about 5! being 120 then I think you have to check your funny bone.

I'm not saying any of the recent posts were particularly funny, but I was just adding to the "comedy" which HoI started.

14. Oct 30, 2013

### Staff: Mentor

People on this forum write all sorts of stuff that seems ridiculous, but that they seriously mean. Since you gave no indication that you were asking with tongue firmly placed in cheek, how was I to know? In your later posts you included the smiley faces, so I could tell your intention.

15. Oct 30, 2013

### skiller

arildno obviously was on the same lines as me. ie we both knew HoI was having a joke.

Nothing more to be said really.

16. Oct 30, 2013

### Staff: Mentor

It's not about whether HoI was being facetious - that was clear to me as well. What I'm saying is that it wasn't clear to me whether you got that joke.

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