Example of a Diverging Series & Converging Integral

[helpmepls]

Homework Statement

Give an example of a continuous, non-negative function f: [1, infinity) --> R such that if an = f(n) for each positive integer n, the series \sum an diverges, while the improper integral from 1 to infinity of f converges. Justify your answer.

Homework Equations

N/A

The Attempt at a Solution

I have tried to randomly pick some functions I thought would work but can't seem to get one to converge and the other to diverge. For instance the series (1/n) will obviously diverge (p-test) but the integral of (1/n) will be the ln(n) which would give me an infinite value when taking the limit as n approaches infinity. Any advice on what type of function would work.

 

[Rudeboy37]

I don't think such function exists.

Proof: Assume such function f(n) exists. Since integral from 1 to infinity converges, the integral test tells us that the sum from 1 to infinity also converges. This contradicts the fact we want the sum to diverge. Therefore no such function exists.

 

[losiu99]

This reasoning applies only to decreasing functions.
Try something which is "large" for integers, but vanishes outside of small intervals around them.

 

[mparks]

This is Dr. Block, please remove this question.