What convergence/divergence test can I use on this series? - Calculus II

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Homework Help Overview

The discussion revolves around determining the convergence or divergence of the series \(\sum\limits_{n = 1}^\infty {\frac{{n!}}{{3n! - 1}}}\) within the context of Calculus II. Participants are exploring various tests to analyze the series.

Discussion Character

  • Exploratory, Assumption checking, Problem interpretation

Approaches and Questions Raised

  • The original poster attempts to apply the Ratio Test but expresses uncertainty about its effectiveness. They also consider using long division to simplify the series. Other participants suggest the Comparison Test and note that the terms do not approach zero, which may indicate divergence.

Discussion Status

The discussion is active, with participants exploring different tests and approaches. Some guidance has been offered regarding the use of the Comparison Test and the implications of terms not approaching zero, but there is no explicit consensus on the best method to apply.

Contextual Notes

Participants are navigating the complexities of convergence tests and are questioning the assumptions underlying their approaches. There is a mention of graphical observations suggesting divergence, but no definitive conclusions have been reached.

opticaltempest
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What test can I use on the following series in order to determine
if it converges or diverges? Looking at it graphically it appears to
diverge but I cannot show it analytically.

[tex] \sum\limits_{n = 1}^\infty {\frac{{n!}}{{3n! - 1}}}[/tex]

Using the Ratio Test, here is I got thus far

[tex] \left| {\frac{{a_{n + 1} }}{{a_n }}} \right| = \left| {\frac{{\left( {\frac{{(n + 1)!}}{{3(n + 1)! - 1}}} \right)}}{{\left( {\frac{{n!}}{{3n! - 1}}} \right)}}} \right|[/tex]

[tex] = \frac{{(n + 1)!}}{{3(n + 1)! - 1}} \cdot \frac{{3n! - 1}}{{n!}}[/tex]

[tex] = \frac{{(n + 1)n!}}{{3(n + 1)n! - 1}} \cdot \frac{{3n! - 1}}{{n!}}[/tex]

[tex] = \frac{{(n + 1)(3n! - 1)}}{{3(n + 1)n! - 1}}[/tex]

At this point, I don't see how the Ratio test will work. What test can I use on this series?

Should I start off with long division to reduce the improper rational
expression?

http://img164.imageshack.us/img164/4656/longdiv7nr.jpg

So the equivalent sum would be

[tex] \sum\limits_{n = 1}^\infty {\left( {\frac{1}{3} + \frac{1}{{3(3n! - 1)}}} \right)} [/tex]

Which must diverge because of the 1/3 always being summed from
1 to infinity.

Is that the correct way to do it? If so, is it the only way? Am I
missing a simple test that could be used?
 
Last edited by a moderator:
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oops nevermind I looked again and it seems u already did that.
 
Maybe comparison test. But the work have looks good to me.
 
I would be inclined to use the fact that is an does not go to 0 then [itex]\Sum a_n[/itex] does not converge!
 

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