Limit Comparison Test: Does L Approaching Infinity Matter?

In summary, The limit comparison test states that if the limit of the ratio of two positive sequences is greater than 0, then the convergence of one sequence is dependent on the convergence of the other. This rule also applies when the limit diverges to infinity. Examples show that if the limit is infinity and the second sequence diverges, then the first sequence also diverges. However, if the second sequence converges, the first sequence may diverge.
  • #1
tmt1
234
0
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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  • #2
Take an example for $a_n,b_n$ and see if the rule satisfies.
 
  • #3
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.
 
  • #4
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 $b_n=1/n^2$.
 

FAQ: Limit Comparison Test: Does L Approaching Infinity Matter?

What is the Limit Comparison Test?

The Limit Comparison Test is a mathematical tool used to determine whether an infinite series converges or diverges. It involves comparing the given series to a known series, typically a p-series, to determine their convergence or divergence.

When should the Limit Comparison Test be used?

The Limit Comparison Test should be used when the given series does not have a recognizable pattern or does not fit any of the other convergence tests. It is also useful when dealing with series that have alternating signs or contain both positive and negative terms.

How does the Limit Comparison Test work?

The Limit Comparison Test works by comparing the given series to a known series, typically a p-series, and taking the limit of their ratio as n approaches infinity. If the limit is a positive finite number, then the given series and the known series either both converge or both diverge. If the limit is 0 or infinity, then the given series and the known series diverge and converge, respectively. If the limit is undefined, then the test is inconclusive.

Does the value of L approaching infinity matter in the Limit Comparison Test?

Yes, the value of L approaching infinity does matter in the Limit Comparison Test. If the limit L is a positive finite number, then the given series and the known series either both converge or both diverge. If L is 0 or infinity, then the given series and the known series diverge and converge, respectively. If L is undefined, then the test is inconclusive.

Can the Limit Comparison Test be used to prove absolute convergence?

Yes, the Limit Comparison Test can be used to prove absolute convergence. If the given series converges absolutely, then it also converges conditionally. This means that if the Limit Comparison Test shows that the given series converges, then it also converges absolutely.

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