Limit Comparison Test: Does L Approaching Infinity Matter?

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SUMMARY

The limit comparison test confirms that if \( L = \lim_{{n}\to{\infty}} \frac{a_n}{b_n} > 0 \), then the convergence or divergence of \( \sum a_n \) directly correlates with \( \sum b_n \). Specifically, if \( L \) diverges to infinity and \( \sum b_n \) diverges, then \( \sum a_n \) also diverges. Conversely, if \( \sum b_n \) converges, the test requires careful examination, as demonstrated with the example \( a_n = 1 \) and \( b_n = \frac{1}{n^2} \), where \( \sum b_n \) converges while \( \sum a_n \) diverges.

PREREQUISITES
  • Understanding of series convergence and divergence
  • Familiarity with limit concepts in calculus
  • Knowledge of the limit comparison test in mathematical analysis
  • Ability to manipulate sequences and series
NEXT STEPS
  • Explore the formal proof of the limit comparison test
  • Investigate additional examples of the limit comparison test with varying sequences
  • Learn about other convergence tests such as the ratio test and root test
  • Study the implications of divergent series in real analysis
USEFUL FOR

Students of calculus, mathematicians, and educators seeking to deepen their understanding of series convergence, particularly those interested in the limit comparison test and its applications.

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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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