How to cancel factorials in power series problems?

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Discussion Overview

The discussion revolves around the cancellation of factorials in power series problems, particularly in the context of applying the ratio test for convergence. Participants are trying to clarify the manipulation of factorial expressions when substituting \( n \) with \( n+1 \).

Discussion Character

  • Technical explanation
  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • One participant expresses confusion about how to cancel factorials when applying the ratio test, specifically questioning the transformation from \([(n+1)!]^2\) to \((n+1)^2(n!)\).
  • Another participant attempts to clarify that \([(n+1)!]^2\) means \([(n+1)!] \times [(n+1)!]\), leading to a more complex expression involving multiple factorial terms.
  • A third participant points out that the initial transformation proposed is incorrect, indicating that \([(n+1)!]^2\) does not equal \((n+1)^2(n!)\) and suggests that this misunderstanding will hinder the simplification process.
  • A later reply corrects a typo in the factorial expression, stating that it should be \([(n+1)!]^2 = [(n+1) \times n!]^2\), providing a clearer path for manipulation.

Areas of Agreement / Disagreement

Participants do not reach a consensus on the correct manipulation of the factorial expression, with some expressing confusion and others providing corrections, indicating that multiple views and interpretations exist.

Contextual Notes

There are unresolved issues regarding the correct interpretation of factorial expressions and the steps involved in their simplification, which may depend on the definitions and assumptions made by participants.

shanepitts
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I have been practicing power series problems and a lot of them include factorials. To find out if they converge or not I'll often use the ratio test. However, I never quite understood how to cancel factorials when replacing the n with n+1. i.e. the textbook has an example problem that shows that

[(n+1)!]2 ⇒ (n+1)2 (n!)

How is this done?

Thank you in advance.
 
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shanepitts said:
I have been practicing power series problems and a lot of them include factorials. To find out if they converge or not I'll often use the ratio test. However, I never quite understood how to cancel factorials when replacing the n with n+1. i.e. the textbook has an example problem that shows that

[(n+1)!]2 ⇒ (n+1)2 (n!)
In your example, [(n+1)!]2 means [(n+1)!] * [(n+1)!], which would be (n + 1)2(n)2(n - 1)2 ... 3222.
shanepitts said:
How is this done?

Thank you in advance.
 
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Note that [(n+1)!]^2 \ne \big(n+1\big)^2 \big(n!\big), so you will have difficulty reducing the left side to the right side. :)
 
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A slight typo, but it should be
shanepitts said:
[(n+1)!]2 ⇒ (n+1)2 (n!)2

(n+1)! = (n+1)\times n!

Hence

\left[ (n+1)!\right]^2 = \left[ (n+1)\times n!\right]^2
 
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Mark44 said:
In your example, [(n+1)!]2 means [(n+1)!] * [(n+1)!], which would be (n + 1)2(n)2(n - 1)2 ... 3222.

Thanks
 

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