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Ch. 10 - Infinite Sequences and Series
Hass - Thomas' Calculus 15th Edition
Hass15th EditionThomas' CalculusISBN: 9780137616077Non è quello che usi tu?Cambia libro di testo
Capitolo 10, Problema 10.4.15

Limit Comparison Test
In Exercises 9–16, use the Limit Comparison Test to determine if each series converges or diverges.
∑ (from n=2 to ∞) 1 / ln n
(Hint: Limit Comparison with ∑ (from n=2 to ∞) (1/n))

Guida verificata passo dopo passo
1
Identify the given series: \( \sum_{n=2}^{\infty} \frac{1}{\ln n} \). We want to determine if this series converges or diverges using the Limit Comparison Test.
Choose a comparison series that is easier to analyze. The hint suggests using \( \sum_{n=2}^{\infty} \frac{1}{n} \), which is a well-known divergent harmonic series.
Set up the Limit Comparison Test by computing the limit \( L = \lim_{n \to \infty} \frac{a_n}{b_n} = \lim_{n \to \infty} \frac{\frac{1}{\ln n}}{\frac{1}{n}} = \lim_{n \to \infty} \frac{n}{\ln n} \).
Analyze the limit \( L \). Since \( n \) grows faster than \( \ln n \), this limit tends to infinity, which is a positive number (not zero or infinite in the sense of the test's conditions).
Interpret the result: Because the limit \( L \) is infinite and the comparison series \( \sum \frac{1}{n} \) diverges, the Limit Comparison Test tells us that the original series \( \sum \frac{1}{\ln n} \) also diverges.

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Limit Comparison Test

The Limit Comparison Test is used to determine the convergence or divergence of an infinite series by comparing it to a second series with known behavior. It involves taking the limit of the ratio of the nth terms of the two series. If the limit is a positive finite number, both series either converge or diverge together.
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Limit Comparison Test

Behavior of the Harmonic Series

The harmonic series ∑ 1/n is a well-known divergent series. Understanding its divergence is crucial when using it as a comparison series in the Limit Comparison Test. Since it diverges, any series that behaves similarly (in terms of term size) will also diverge.
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P-Series and Harmonic Series

Properties of the Natural Logarithm Function

The natural logarithm function ln(n) grows slowly as n increases, but it still tends to infinity. Recognizing how 1/ln(n) compares to 1/n helps in applying the Limit Comparison Test. Since ln(n) grows slower than n, 1/ln(n) decreases more slowly than 1/n, affecting convergence.
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Properties of Functions