Convergence of \sum_{n=1}^\infty \frac{1}{n!}: A Basic Comparison Test

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SUMMARY

The discussion centers on the convergence of the series \(\sum_{n=1}^\infty \frac{1}{n!}\) and the divergence of \(\sum_{n=1}^\infty \frac{2^n}{n^2}\). Participants confirm that the first series converges, suggesting comparisons to p-series, specifically \(\frac{1}{n^2}\). For the second series, it is established that it diverges due to the dominance of the exponential term \(2^n\) over the polynomial term \(n^2\), with recommendations to utilize the limit comparison test for proof.

PREREQUISITES
  • Understanding of factorial notation and its implications in series convergence
  • Familiarity with p-series and their convergence criteria
  • Knowledge of the limit comparison test for series
  • Basic concepts of exponential growth versus polynomial growth
NEXT STEPS
  • Study the Limit Comparison Test in detail to apply it effectively in series analysis
  • Explore the properties of p-series, particularly the conditions for convergence
  • Investigate the behavior of exponential functions compared to polynomial functions
  • Learn about the Ratio Test and its application in determining series convergence
USEFUL FOR

Mathematics students, educators, and anyone studying series convergence, particularly those interested in advanced calculus and analysis techniques.

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\sum_{n=1}^\infty \frac{1}{n!}

I understand what n! means, but I have no clue what to compare this to. It is obvious to me that the sum converges, but I'm not sure how to prove it. I assume I would compare it to a p-series but I need help. Thanks!
 
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Compare it to 1/n^2...
 
micromass said:
Compare it to 1/n^2...

lol, I just realized how simple this is. my bad.
 
Sorry, one more.

\sum_{n=1}^\infty \frac{2^n}{n^2}

What would I compare this to?

I can clearly see that it diverges since numerator is waaaay bigger but I don't know how to prove it.
 
Calculate the limit of the terms and show that the limit isn't 0.
 
micromass said:
Calculate the limit of the terms and show that the limit isn't 0.

Sorry I didn't specify. I understand how to use the limit test. For this problem I am supposed to compare it to something. Thanks for putting up with my questions :).

edit: since 2^n is soo much bigger then n^2 can I compare it to 2^n?
 
Maybe the harmonic series??

It's a stupid exercise anyway if you're not allowed to do the limit test.
 

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