Question 1Multiple choiceSequences & series convergence
Consider the series
Which statement is correct?
- A.
The series converges absolutely.
- B.
The series converges conditionally.
- C.
The series diverges by the limit comparison test with .
- D.
The series diverges because its terms do not approach 0.
Worked solution
Write . Apply the alternating series test, which needs and decreasing.
Limit: , so the terms do go to zero and option D is ruled out.
Decreasing: for , the quotient rule gives
so for all . Both conditions hold, so the alternating series converges.
Absolute convergence: the series of absolute values is . Limit comparison with the harmonic series :
a finite positive number, and diverges, so diverges. Hence the series does not converge absolutely.
A series that converges while its series of absolute values diverges is, by definition, conditionally convergent.
The tempting wrong answer is C: the limit comparison test is carried out correctly, but it only tells us about . Divergence of the absolute series never implies divergence of the original alternating series; that is exactly the situation the word "conditionally" describes.
The series converges conditionally (option B).