Find the radius of convergence for: (n!)?r" Σ (2n)! n=1

Advanced Engineering Mathematics
10th Edition
ISBN:9780470458365
Author:Erwin Kreyszig
Publisher:Erwin Kreyszig
Chapter2: Second-order Linear Odes
Section: Chapter Questions
Problem 1RQ
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**Problem Statement:**

Find the radius of convergence for the series:

\[
\sum_{n=1}^{\infty} \frac{(n!)^2 x^n}{(2n)!}
\]

**Instructions:**

- Use appropriate convergence tests to determine the radius of convergence.
- Input your final answer in the provided box.

--------------------------------------------------------------------

**Detailed Explanation:**

The series given is:

\[
\sum_{n=1}^{\infty} \frac{(n!)^2 x^n}{(2n)!}
\]

To find the radius of convergence, consider using the ratio test or the root test. This requires examining the behavior of the terms as \( n \to \infty \). 

The ratio test considers:

\[
\lim_{n \to \infty} \left| \frac{a_{n+1}}{a_n} \right|
\]

where \( a_n = \frac{(n!)^2 x^n}{(2n)!} \).

Calculate:

\[
a_{n+1} = \frac{((n+1)!)^2 x^{n+1}}{(2(n+1))!}
\]

Substitute and simplify the following expression for the ratio test:

\[
\left| \frac{a_{n+1}}{a_n} \right| = \left| \frac{((n+1)!)^2 x^{n+1}}{(2(n+1))!} \cdot \frac{(2n)!}{(n!)^2 x^n} \right|
\]

Finally, solve the limit as \( n \to \infty \) to determine the radius of convergence.
Transcribed Image Text:**Problem Statement:** Find the radius of convergence for the series: \[ \sum_{n=1}^{\infty} \frac{(n!)^2 x^n}{(2n)!} \] **Instructions:** - Use appropriate convergence tests to determine the radius of convergence. - Input your final answer in the provided box. -------------------------------------------------------------------- **Detailed Explanation:** The series given is: \[ \sum_{n=1}^{\infty} \frac{(n!)^2 x^n}{(2n)!} \] To find the radius of convergence, consider using the ratio test or the root test. This requires examining the behavior of the terms as \( n \to \infty \). The ratio test considers: \[ \lim_{n \to \infty} \left| \frac{a_{n+1}}{a_n} \right| \] where \( a_n = \frac{(n!)^2 x^n}{(2n)!} \). Calculate: \[ a_{n+1} = \frac{((n+1)!)^2 x^{n+1}}{(2(n+1))!} \] Substitute and simplify the following expression for the ratio test: \[ \left| \frac{a_{n+1}}{a_n} \right| = \left| \frac{((n+1)!)^2 x^{n+1}}{(2(n+1))!} \cdot \frac{(2n)!}{(n!)^2 x^n} \right| \] Finally, solve the limit as \( n \to \infty \) to determine the radius of convergence.
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