Example of a Diverging Series & Converging Integral

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Homework Help Overview

The discussion revolves around finding a continuous, non-negative function defined on the interval [1, infinity) that results in a diverging series while the corresponding improper integral converges. The original poster is exploring examples of such functions and seeking guidance on their properties.

Discussion Character

  • Exploratory, Assumption checking

Approaches and Questions Raised

  • The original poster attempts to identify functions that meet the criteria but struggles to find a suitable example. They mention the series (1/n) as a diverging case and note the integral's behavior.
  • Some participants question the existence of such a function, citing the integral test and its implications for convergence.
  • Others suggest considering functions that exhibit specific behaviors, such as being large at integers but diminishing elsewhere.

Discussion Status

The discussion is ongoing, with various perspectives being explored. While some participants assert that no such function can exist based on the integral test, others propose alternative approaches to finding a suitable function. There is no explicit consensus on the existence of the function, and the conversation remains open to further exploration.

Contextual Notes

Participants are navigating the constraints of the problem, including the definitions of convergence and divergence in the context of series and integrals. The original poster's attempts are influenced by their understanding of these concepts, and assumptions about function behavior are being examined.

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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.
 
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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.
 


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


This is Dr. Block, please remove this question.
 

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