Why does this infinite series diverge?

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SUMMARY

The series \(\sum_{n=1}^{\infty} \frac{(-1)^{(2n+2)}}{n+1}\) diverges due to the application of the Direct Comparison Test. Although the numerator simplifies to 1 for all \(n\), the denominator grows without bound, leading to a divergent series when compared to the harmonic series \(\sum_{n=1}^{\infty} \frac{1}{n}\). The confusion arose from misapplying the Alternating Series Test, as the series is not alternating. The correct interpretation involves recognizing that the series behaves like a divergent p-series.

PREREQUISITES
  • Understanding of series convergence tests, specifically the Direct Comparison Test and the Alternating Series Test.
  • Familiarity with p-series and their convergence criteria.
  • Basic knowledge of limits and how they apply to series.
  • Ability to manipulate and simplify mathematical expressions involving exponents.
NEXT STEPS
  • Study the Direct Comparison Test in detail to understand its application in determining series convergence.
  • Learn about p-series and their properties, focusing on conditions for convergence and divergence.
  • Explore the Integral Test for convergence, particularly for series resembling logarithmic functions.
  • Review the Alternating Series Test and its specific requirements for series to be classified as alternating.
USEFUL FOR

Students studying calculus, particularly those focusing on series and convergence tests, as well as educators seeking to clarify common misconceptions about series behavior.

kmacinto
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Homework Statement


Why does this series diverge?

Homework Equations


\sum_{n}^{\infty }\frac{-1^{(2n+2)}}{n+1}

The Attempt at a Solution



I must be missing a rule with the -1 sign.

My logic is that for all n, the numerator = 1 since anything to the power of 2 is even and adding the 2 doesn't change the sign either. So if the numerator is always 1 and the denominator grows without bound so by the alternating series test since, L = 0 and the series is decreasing, the series should converge... My math book and maple says different though :(
 
Last edited:
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kmacinto said:

Homework Statement


Why does this series diverge?


Homework Equations


\sum_{n}^{\infty }\frac{-1^{(2n+2)}}{n+1}


The Attempt at a Solution



I must be missing a rule with the -1 sign.

My logic is that for all n, the numerator = 1 since anything to the power of 2 is even and adding the 2 doesn't change the sign either. So if the numerator is always 1 and the denominator grows without bound so by the alternating series test since, L = 0 and the series is decreasing, the series should converge... My math book and maple says different though :(
As you pointed out, it's not an alternating series.

For clarity, put parentheses around the -1. _ _ _ _ (-1)

\displaystyle \sum_{n}^{\infty }\frac{(-1)^{(2n+2)}}{n+1}
 
kmacinto said:

Homework Statement


Why does this series diverge?

Homework Equations


\sum_{n}^{\infty }\frac{-1^{(2n+2)}}{n+1}

The Attempt at a Solution



I must be missing a rule with the -1 sign.

My logic is that for all n, the numerator = 1 since anything to the power of 2 is even and adding the 2 doesn't change the sign either. So if the numerator is always 1 and the denominator grows without bound so by the alternating series test since, L = 0 and the series is decreasing, the series should converge... My math book and maple says different though :(

S=Ʃ(-1)2k+2/(k+1)
Try a simpler case of the above. Try the other tests. Maybe the integral test esp. something looking like a log fn differentiated.
 
Last edited:
I think I have it... If the numerator is positive for all n, I can just use the direct comparison test with B sub N = 1/n which is a divergent p-series and since B sub N is less than A sub N, A sub N also diverges... right?
 
That sounds good to me !
 

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