Why Does the Limit of Sine at Infinity Confuse Mathematicians?

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Homework Help Overview

The discussion revolves around the limit of the sine function as its argument approaches infinity, specifically the expression \(\lim_{x \rightarrow \infty} \sin \left( 2\pi \sqrt{x^2+1} \right)\). Participants express confusion regarding whether this limit exists or equals zero, raising questions about the behavior of the sine function and the nature of the argument as \(x\) increases.

Discussion Character

  • Exploratory, Assumption checking, Conceptual clarification

Approaches and Questions Raised

  • Some participants attempt to reason that as \(x\) becomes very large, \(\sqrt{x^2+1}\) approaches an integer, leading to the conclusion that \(\sin(2\pi n) = 0\) for integers \(n\). Others question the validity of assuming \(x\) is an integer and explore the implications of treating \(x\) as a real number. There are discussions about the periodic nature of the sine function and whether the limit can be defined under different assumptions about \(x\).

Discussion Status

The discussion is ongoing, with various interpretations being explored. Some participants suggest that the limit might not exist due to the periodicity of the sine function, while others propose that if \(x\) is restricted to integers, the limit could be zero. There is no explicit consensus, but productive dialogue continues around the assumptions and definitions involved.

Contextual Notes

Participants note the ambiguity in the original problem statement regarding whether \(x\) is to be considered as an integer or a real number. This distinction significantly impacts the conclusions drawn about the limit's existence and value.

twoflower
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Hi,

I've been trying to compute

[tex] \lim_{x \rightarrow \infty} \sin \left( 2\pi \sqrt{x^2+1} \right)[/tex]

But don't have any idea. According to the results it should be 0, but I can't see the way I should get it...

Thank you.
 
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for x>>1.. the square root approches to an integer... (even if it is going to infinite)
 
vincentchan said:
for x>>1.. the square root approches to an integer... (even if it is going to infinite)

Yes, the square root approaches to infinity I think, but I still don't get it.
 
it is also an integer... sin(2pi n) =0
 
vincentchan said:
it is also an integer... sin(2pi n) =0

Well, that's the thought I got instantly after the first look, but I'm trying to produce some serious proof of that...If it were in test, do you think that your answer would be accepted?
 
The limit should be undefined. Sine is periodic and there is no way to define what's going to happen when you take such a limit.

If x is defined to be a "very large" integer, I can see an argument for the answer zero, but by default x should be taken as real, and there is no defined limit.
 
vincentchan said:
for x>>1.. the square root approches to an integer... (even if it is going to infinite)

That's only if you assume x to be an integer, and there's no basis for such an assumption.

What if [itex]x^2 = e^2(10^{16}) - 1[/itex], for example ?
 
because he said the result is zero... it is reasonable for me to make an assumtion that x is integer... otherwise... obviously, the function is undefine
 
vincentchan said:
because he said the result is zero... it is reasonable for me to make an assumtion that x is integer... otherwise... obviously, the function is undefine

But what 'results' is he talking about ? The natural tendency is to keep plugging in large integer values (like 10^16) into the calculator in which case of course, you'd get zero. It would also be wrong.
 
  • #10
I second Curious,

The function is well-defined, but the particular limit does not exist.

Twoflower,

How did you get these "results" ?
 
  • #11
Gokul43201 said:
I second Curious,

The function is well-defined, but the particular limit does not exist.

Twoflower,

How did you get these "results" ?

He gave us these limits as recommended homework along with the results...The only that comes to my mind is that I miswrote the original limit and replaced "n" with "x" :-(
 
  • #12
Okay,if it was "n" instead of "x",then the limit still does not exist.Moreover,it can be even shown it cannot be zero.For it to be zero,it is necessary that
[tex]\lim_{n\rightarrow +\infty} \sqrt{n^{2}+1} =\ natural \ multiple \ of \frac{1}{2}[/tex]

Now u have an equation to solve:are there natural "n"-s for which
[tex]\sqrt{n^{2}+1}=\frac{k}{2}[/tex]
,for arbitrary "k",very large?
My answer is "no",therefore the limit cannot be "0".Moreover,it doesn't exist.

Daniel.
 
  • #13
dextercioby said:
Okay,if it was "n" instead of "x",then the limit still does not exist.Moreover,it can be even shown it cannot be zero.For it to be zero,it is necessary that
[tex]\lim_{n\rightarrow +\infty} \sqrt{n^{2}+1} =\ natural \ multiple \ of \frac{1}{2}[/tex]

Now u have an equation to solve:are there natural "n"-s for which
[tex]\sqrt{n^{2}+1}=\frac{k}{2}[/tex]
,for arbitrary "k",very large?
My answer is "no",therefore the limit cannot be "0".Moreover,it doesn't exist.

Daniel.

Thank you Daniel but I can't see what couldn't the whole expression in sin be natural multiple of [itex]\pi[/itex] (and ie. [itex]\sqrt{n^2+1}[/itex] any integer)...
 
  • #14
Well,solve this equation
[tex]\sqrt{n^{2}+1}=k[/tex]
for natural "k" and "n".And pick out the (possible,if any) solutions,the ones for very large"n".

Daniel.
 
  • #15
you confused the concept of limit again... [tex]\sqrt{n^{2}+1}[/tex] approach to an integer while n is large... but [tex]\sqrt{n^{2}+1}[/tex] itself is not an integer... proof:
[tex]\lim_{n \rightarrow \infty} \sqrt{n^{2}+1} = n[/tex] where n is an integer... done
[tex] \lim_{n \rightarrow \infty} \sin \left( 2\pi \sqrt{n^2+1} \right) = 0[/tex]
sure, I have made an assumtion that n is integer...for an abitrary real number, the limit doesn't exist...
 
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  • #16
So what's your point??Is that limit zero,or it doesn't exist?? :confused:

Daniel.
 
  • #17
if n is integer, the limit exist, if not,then not...
but you do agree with me that [itex]\lim_{x \rightarrow \infty} \sqrt{x^2+1}[/itex] is integer, right?
 
  • #18
vincentchan said:
if n is integer, the limit exist, if not,then not...
but you do agree with me that [itex]\lim_{x \rightarrow \infty} \sqrt{x^2+1}[/itex] is integer, right?

Yes, this seems more logical to me than the Daniel's one...
 
  • #19
vincentchan said:
if n is integer, the limit exist, if not,then not...
but you do agree with me that [itex]\lim_{x \rightarrow \infty} \sqrt{x^2+1}[/itex] is integer, right?

No. The limit doesn't exist, so it can't be an integer. :-p

However, [itex]\lim_{x \rightarrow \infty} \mathrm{frac} \sqrt{x^2+1} = 0[/itex]

If x is only allowed to range over integers. (frac is the fractional part, so frac 1.2 = .2)
 
  • #20
vincentchan said:
if n is integer, the limit exist, if not,then not...
but you do agree with me that [itex]\lim_{x \rightarrow \infty} \sqrt{x^2+1}[/itex] is integer, right?

If [tex]+\infty[/tex] is an integer,i agree.

Can u show that [tex]+\infty[/tex] is an integer?

Daniel.
 
  • #21
Hurkyl agrees with me,so i must be right... :-p

Daniel.

EDIT:Nice quote... :smile: :smile: :smile:
 
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  • #22
dextercioby said:
Can u show that LaTeX graphic is being generated. Reload this page in a moment. is an integer?
this is the assumtion i made earlier... no need to show
 
  • #23
Hurkyl said:
No. The limit doesn't exist, so it can't be an integer. :-p

However, [itex]\lim_{x \rightarrow \infty} \mathrm{frac} \sqrt{x^2+1} = 0[/itex]

If x is only allowed to range over integers. (frac is the fractional part, so frac 1.2 = .2)

Is there a proper mathematical basis for saying "x is only allowed to range over integers" for a limit ? I'm not referring to this question in particular, but can you say that for a general limit, e.g. does the statement

[tex]\lim_{\substack{x \rightarrow \infty}} f(x)[/tex], where [tex]x \in {Z^+}[/tex]

have valid mathematical meaning ? Thanks.
 
  • #24
That is a deficiency in notation -- the domain is left implicit. It's one of those things you're supposed to infer from context (and will be stated explicitly when such an inference can't be made)

Incidentally, you'll usually see n or m as the dummy variable when it's supposed to range over integers.
 
  • #25
first show there exist infinitely many positive integer (using induction),
by the reason above, it is reasonable to say a positive integer x approach to infinite..
 
  • #26
Hurkyl said:
That is a deficiency in notation -- the domain is left implicit. It's one of those things you're supposed to infer from context (and will be stated explicitly when such an inference can't be made)

Incidentally, you'll usually see n or m as the dummy variable when it's supposed to range over integers.

If the original question is restated allowing x to vary only over the integers, might the limit of zero be correct ? The fractional part tends to zero after all.
 
  • #27
Well,then yes.It would be
[tex]\lim_{n\rightarrow +\infty} \sin(2\pi[\sqrt{n^{2}+1}]) =0[/tex]

Daniel.
 
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  • #28
dextercioby said:
Well,then yes.It would be
[tex]\lim_{n\rightarrow +\infty} \sin(2\pi[\sqrt{n^{2}+1}]) =0[/tex]

Daniel.

Ok I think the original limit was supposed to be a limit of sequence, but then I think it doesn't exist anyway...
 
  • #29
dextercioby said:
Well,then yes.It would be
[tex]\lim_{n\rightarrow +\infty} \sin(2\pi[\sqrt{n^{2}+1}]) =0[/tex]

Daniel.


So you've changed your mind from this previous post ?

dextercioby said:
Okay,if it was "n" instead of "x",then the limit still does not exist.Moreover,it can be even shown it cannot be zero.For it to be zero,it is necessary that
[tex]\lim_{n\rightarrow +\infty} \sqrt{n^{2}+1} =\ natural \ multiple \ of \frac{1}{2}[/tex]

Now u have an equation to solve:are there natural "n"-s for which
[tex]\sqrt{n^{2}+1}=\frac{k}{2}[/tex]
,for arbitrary "k",very large?
My answer is "no",therefore the limit cannot be "0".Moreover,it doesn't exist.

Daniel.

Then we're in agreement now. For x ranging only over the integers, the limit is zero. For real x, there is no limit.
 
  • #30
I finally have it. I didn't become aware that if "n" goes over integers only, it really has the limit zero.
 

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