Difference between lim as x→∞ and lim as |x|→∞

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Discussion Overview

The discussion centers around the differences between the limits as \( x \to \infty \) and \( |x| \to \infty \), particularly in the context of the function \( x \sin \frac{1}{x} \). Participants explore the implications of using absolute values in limit notation and whether this affects the evaluation of limits, considering both real and complex variables.

Discussion Character

  • Exploratory
  • Technical explanation
  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • Some participants suggest that using \( |x| \to \infty \) implies that the limits as \( x \to \infty \) and \( x \to -\infty \) are equal.
  • Others note that limits as \( x \to \pm \infty \) are sometimes used to convey similar meanings, indicating a potential equivalence in certain contexts.
  • A participant introduces the idea of interpreting \( |x| \to \infty \) as considering \( x \) in a complex plane, where the limit approaches a fixed value regardless of direction.
  • Another viewpoint emphasizes that the notation \( |x| \to \infty \) is more meaningful when \( x \) is treated as a complex variable, while \( x \to \infty \) is applicable for real numbers approaching positive infinity.
  • One participant mentions observing symmetry in the graph of \( x \sin \frac{1}{x} \) and notes that both limits equal 1, suggesting a relationship between the graphical behavior and the limit notation.

Areas of Agreement / Disagreement

Participants express differing interpretations of the notation \( |x| \to \infty \), with no clear consensus on its implications or the equivalence of the limits involved. The discussion remains unresolved regarding the precise meaning and application of these limit notations.

Contextual Notes

Some participants reference specific mathematical definitions and concepts related to limits, but these are not universally accepted or agreed upon within the discussion. The implications of using absolute values in limits are not fully explored, leaving room for further clarification.

Cristopher
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I came across something I'd never seen before, the use of |x| instead of just ‘x’ in limits.
What is the difference between [itex]\displaystyle\lim_{|x|\to\infty}x\sin\frac{1}{x}[/itex] and [itex]\displaystyle\lim_{x\to\infty}x\sin\frac{1}{x}[/itex] ?
Is there any difference when evaluating them? Is that notation used only with infinity?

Thanks.
 
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It seems like it's implying that the limit as x goes to infinity is equal to the limit as x goes to negative infinity. But someone who has actually seen this notation before might know better, I'm just guessing.
 
I haven't seen limits as |x|→∞, but I have seen limits as x→±∞ or limits as x→∞ with this meaning. In the latter case they distinguished limits as x→+∞ and as x→-∞. I think they pictured ∞ as a single point outside the line, making it a circle, its Alexandroff compactification, considering limits as x→+∞ and as x→-∞ as the lateral limits towards ∞.

About the limits as |x|→∞, I think it could be generalized in the following way. We may say that f(x)→a as g(x)→b if, for any neighborhood U of a, there exists a neighborhood V of b such that f(g-1(V\{b}))⊆U. Also f(x)→a as g(x)→∞ if, for any neighborhood U of a, there exists M>0 such that f(g-1((M,∞)))⊆U, and similar definitions.
 
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I see two different ways of interpreting "limit as |x| goes to infinity". First would be that "limit as x goes to infinity" and"limit as x goes to -infinity" must be the same. The other would be that x represents a point in the plane or a complex number and x goes away from the origin in any direction.
 
Cristopher said:
What is the difference between [itex]\displaystyle\lim_{|x|\to\infty}x\sin\frac{1}{x}[/itex] and [itex]\displaystyle\lim_{x\to\infty}x\sin\frac{1}{x}[/itex] ?
Is there any difference when evaluating them? Is that notation used only with infinity?
The first notation only really makes sense if you consider 'x' to be a complex variable. In this case, taking the limit at infinity means asking whether the function approaches a fixed value no matter which direction you approach infinity from.

The second notion on the other hand can be used if x is real and you are simply going to "positive" infinity.

At infinity, functions of complex variables generally either have a finite limit, or else a pole. They could also have an essential singularity (O~o) as well. And if the function is multi-valued, it will almost always have a (irregular) branch point there.
 
Thank you all.

Yes, I also thought that it says that the limit as x→+∞ and as x→-∞ are the same. Indeed, I did look at the graph of the funcion x sin(1/x) before asking, and these limits are both equal to 1. I also noticed there's symmetry in the graph, I thought it may have something to do with that, too. I posted the question in the hope of getting more info on the scope of this notation.

Just for reference I saw the notation here:
http://en.wikipedia.org/wiki/L%27Hopital%27s_rule
In the section ‘Other ways of evaluating limits’
 

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