Solution

ID: past-exam-of-the-mathematics-course-of-the-year-1-of-the-natural-sciences-course-of-the-university-of-cambridge/2010/paper-1/20/b/solution

For absolute convergence, examine . When , the ratio of successive terms tends to , so the ratio test proves convergence; at every term is zero. For , the terms do not tend to zero.
At either boundary , the absolute-value series is the divergent harmonic series. Thus for real the complete answer is
The original series at is conditionally convergent by the alternating-series criterion, but it is not absolutely convergent and is correctly excluded here.

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