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Ch. 11 - Power Series
Briggs - Calculus: Early Transcendentals 3rd Edition
Briggs3rd EditionCalculus: Early TranscendentalsISBN: 9780136847243Non è quello che usi tu?Cambia libro di testo
Capitolo 11, Problema 11.2.11

Radius and interval of convergence Determine the radius and interval of convergence of the following power series.


∑ₖ₌₁∞ (kx)ᵏ

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Identify the given power series: \( \sum_{k=1}^{\infty} (kx)^k \). This can be rewritten as \( \sum_{k=1}^{\infty} k^k x^k \).
To find the radius of convergence, use the root test which involves the limit \( L = \lim_{k \to \infty} \sqrt[k]{|a_k|} \), where \( a_k = k^k x^k \).
Calculate \( \sqrt[k]{|a_k|} = \sqrt[k]{k^k |x|^k} = k |x| \).
Find the limit \( L = \lim_{k \to \infty} k |x| \). For the series to converge, this limit must be less than 1, so set \( \lim_{k \to \infty} k |x| < 1 \).
Since \( \lim_{k \to \infty} k |x| = \infty \) for any \( x \neq 0 \), the radius of convergence is 0, meaning the series converges only at \( x = 0 \). The interval of convergence is therefore \( \{0\} \).

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Power Series

A power series is an infinite sum of terms in the form a_k(x - c)^k, where a_k are coefficients and c is the center. Understanding the structure of power series is essential to analyze their convergence behavior depending on the variable x.
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Intro to Power Series

Radius of Convergence

The radius of convergence is the distance from the center c within which the power series converges absolutely. It can be found using tests like the Ratio Test or Root Test, and it defines the interval where the series behaves well.
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Radius of Convergence

Interval of Convergence

The interval of convergence is the set of all x-values for which the power series converges. It includes all points within the radius of convergence and requires checking endpoints separately to determine if the series converges there.
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Interval of Convergence
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