Determine whether the limit exists and evaluate the integral if it does

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Homework Help Overview

The discussion revolves around determining the existence of a limit related to an integral involving the function \(\frac{1}{\sqrt[3]{1-x^{2n}}}\) over the interval \([-1, 1]\). Participants are exploring the application of the Dominated Convergence Theorem and considering power series representations in the context of this limit.

Discussion Character

  • Exploratory, Assumption checking, Mathematical reasoning

Approaches and Questions Raised

  • Participants are attempting to identify a dominating function for the integral and are questioning the relevance of the variable \(k\) in the limit. There is discussion about the relationship between \(x^{2n}\) and \(x^2\) within the specified interval, as well as the implications of this relationship for the integral's evaluation.

Discussion Status

The conversation has progressed with some participants suggesting potential dominating functions and others expressing uncertainty about their applicability. There is acknowledgment of a previously overlooked function that may assist in the evaluation of the integral, indicating a productive direction in the discussion.

Contextual Notes

Participants are navigating through potential constraints related to the integrability of certain functions over the interval \([-1, 1]\) and the implications of using the Dominated Convergence Theorem in this context.

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Homework Statement



Determine whether the limit

[itex]\lim_{n \rightarrow \infty} \int_{-1}^1 \frac{1}{\sqrt[3]{1-x^{2n}}} dx[/itex]

exists and evaluate the integral if it does

Homework Equations



Dominated convergence theorem (I think) and a power series representation.

The Attempt at a Solution



I've been attempting to find a dominating function for [itex]\frac{1}{\sqrt[3]{1-x^{2n}}}[/itex] by looking at the power series representation for [itex]\frac{1}{1-x^{2n}}[/itex], but I'm not seeing it. I'd appreciate any help.
 
Last edited:
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resolvent1 said:

Homework Statement



Determine whether the limit

[itex]\lim_{k \rightarrow \infty} \int_{-1}^1 \frac{1}{\sqrt[3]{1-x^{2n}}} dx[/itex]

exists and evaluate the integral if it does


Homework Equations



Dominated convergence theorem (I think) and a power series representation.


The Attempt at a Solution



I've been attempting to find a dominating function for [itex]\frac{1}{\sqrt[3]{1-x^{2n}}}[/itex] by looking at the power series representation for [itex]\frac{1}{1-x^{2n}}[/itex], but I'm not seeing it. I'd appreciate any help.
What does k have to do with this?

Why take the limit as k → ∞ ?
 
Much more interesting if we consider:

[tex]\lim_{n\to\infty}\int_{-1}^1 \frac{dx}{\sqrt[3]{1-x^{2n}}}[/tex]

and in that case isn't:

[tex]x^{2n}\leq x^2[/tex]

in that interval?
 
Last edited:
Sorry, that should be an n, not a k.

It's true that x^{2n} \leq x^2 on [-1,1], so [itex]\frac{1}{\sqrt[3]{1-x^{2n}}} \leq \frac{1}{1-x^2}[/itex] on [-1,1], but [itex]\frac{1}{1-x^2}[/itex] isn't in [itex]L^1(-1,1)[/itex]

So I'm not sure what to do with it.
 
What about ##1 \over \sqrt{1-x^2}##?
 
Thanks, I think that works perfectly. (I can't believe I didn't see that before - sorry - and that problem was really starting to piss me off.)
 
Good! :)

Does that mean that you also evaluated the integral in its limit?
 
Yeah, I knew that several hours ago, just not the dominating function. THank you for your help.
 

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