Unsure of solution to improper integral

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Discussion Overview

The discussion revolves around the evaluation of the improper integral ∫[∞][1] ln(x) x^-1 dx. Participants explore methods for determining whether the integral converges or diverges, including the use of substitution and the comparison test.

Discussion Character

  • Technical explanation, Mathematical reasoning, Homework-related

Main Points Raised

  • One participant expresses uncertainty about proving divergence, questioning whether an answer of ∞ suffices or if a comparison test is necessary.
  • Another participant suggests calculating the integral from 1 to Y and taking the limit as Y approaches infinity to demonstrate that the integral does not exist.
  • A third participant mentions that the integral has a simple antiderivative after substitution and supports the previous suggestion of taking the limit as Y approaches infinity.

Areas of Agreement / Disagreement

Participants generally agree on the approach of evaluating the integral and taking limits, but there is no consensus on the necessity of the comparison test to prove divergence.

Contextual Notes

Some assumptions about the convergence of the integral may be implicit, and the discussion does not resolve whether the comparison test is required for establishing divergence.

Satirical T-rex
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I've been trying to solve this improper integral ∫[∞][1] ln(x) x^-1 dx. I couldn't find any way to use the comparison test to find divergence, so I used substitution and got ∞-∞ which I was pretty sure was divergence until I noticed I put 0 instead of 1 making my answer ∞. Do I need to prove divergence with a comparison test or is an answer of ∞ enough to prove it.
 
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Hi T, :welcome:
You could work out $$\int_1^Y {\ln x\over x} dx $$ (As I think you did already) and take ##\lim Y\rightarrow \infty## to show the integral does not exist.
 
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Hi,this integral has simple antiderivative (after substituting ##\ln##), after you can take the limit of the result for ##Y\rightarrow +\infty## (as suggested by @BvU ).
 
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Thanks for your help and the warm welcome.
 
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