Limit Comparison Test: Does L Approaching Infinity Matter?

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Discussion Overview

The discussion revolves around the limit comparison test in the context of series convergence, specifically examining the implications when the limit \( L \) approaches infinity. Participants explore whether the test's conclusions hold under these conditions, with a focus on theoretical understanding and examples.

Discussion Character

  • Exploratory
  • Mathematical reasoning

Main Points Raised

  • One participant states the limit comparison test and questions if it applies when \( L \) diverges to infinity.
  • Several participants suggest testing specific examples of \( a_n \) and \( b_n \) to investigate the validity of the test under these conditions.
  • Another participant proposes that if \( a_n > 0 \) and \( b_n > 0 \) with \( \lim_{n\to\infty} \frac{a_n}{b_n} = \infty \), then \( \sum b_n \) diverging implies \( \sum a_n \) also diverges, but raises a counterexample for when \( \sum b_n \) converges.

Areas of Agreement / Disagreement

Participants do not reach a consensus on the applicability of the limit comparison test when \( L \) approaches infinity, with differing views and examples presented that suggest both support and challenge the test's conclusions.

Contextual Notes

Some examples provided may not cover all necessary conditions or assumptions, and there is a lack of resolution regarding the implications of \( L \) diverging to infinity.

tmt1
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The limit comparison test states that if $a_n$ and $b_n$ are both positive and $L = \lim_{{n}\to{\infty} } \frac{a_n}{b_n} > 0$ then $\sum_{}^{} a_n$ will converge if $\sum_{}^{} b_n$ and $\sum_{}^{} a_n$ will diverge if $\sum_{}^{} b_n$ diverges. Does this rule also apply if $L$ diverges to infinity?
 
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Take an example for $a_n,b_n$ and see if the rule satisfies.
 
ZaidAlyafey said:
Take an example for $a_n,b_n$ and see if the rule satisfies.

It seems to be true based on the examples I've tried, but I'm not sure if I've tried enough examples.
 
If $a_n>0$ and $b_n>0$, $\lim_{n\to\infty} \frac{a_n}{b_n}=\infty$ and $\sum b_n$ diverges, then $\sum a_n$ also diverges. But for the case when $\sum b_n$ is convergent, take $a_n=1$ and $b_n=1/n^2$.
 

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