Q1. Does a series sum_infty^{n=1}bn converge if bnrightarrow 0? Justify your answer by at least two examples?

Ramsey

Ramsey

Answered question

2021-03-05

Q1. Does a series n=1bn converge if bn0? Justify your answer by at least two examples?

Answer & Explanation

jlo2niT

jlo2niT

Skilled2021-03-06Added 96 answers

Here are a couple of easier examples so that you get a better handle on this.
A geometric series
Consider for example bn=12n. Clearly bn0 as n. Then the series
n=1bn
This result can be shown geometrically (hence the name of this type of series) or by rewriting the series as a telescoping sum. So clearly this series satisfies the given condition and converges.
The harmonic series
Now consider the very simple looking example bn=1n. Again, it should be easier to see that bn0 as n. However, this time the series diverges to infinity. This can be shown using either the comparison test or the integral test.
Conclusion
Thus the requirement that bn0 is not a strong enough requirement to let us know whether a series converges or not. There are examples where it does and examples where it doesn't.

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