Real Analysis Question regarding Series

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Homework Help Overview

The problem involves a monotonically decreasing sequence of positive real numbers, {a_n}, with the limit approaching zero. The task is to demonstrate that the radius of convergence of the series \(\sum a_n x^n\) is at least 1.

Discussion Character

  • Exploratory, Assumption checking

Approaches and Questions Raised

  • The original poster expresses uncertainty about how to begin the proof and requests guidance on outlining the approach. Another participant suggests considering the limit definition of the radius of convergence and questions the necessity of the condition that lim a_n = 0.

Discussion Status

The discussion includes attempts to clarify the implications of the sequence's properties on the radius of convergence. Some participants are exploring the significance of the limit condition, while others are providing insights into the limit calculation for the radius of convergence.

Contextual Notes

There is a lack of explicit consensus on the necessity of the condition that lim a_n = 0, and the original poster has not made significant progress in their attempt to solve the problem.

christianrhiley
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Homework Statement



Let {a_n} be a monotonically decreasing sequence of positive real numbers with lim a_n = 0. Show the radius of convergence of [tex]\sum[/tex]a_nx[tex]^{}n[/tex] is at least 1.


The Attempt at a Solution


I have no real attempt at a solution since I'm unsure how to proceed. I've tried using the power series definition and using a fixed x_0, but I get nowhere using this method. Can someone outline how this proof might look? Thanks.
 
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The radius of convergence is given by
[tex]\lim_{n \to \infty} \left| \frac{a_n}{a_{n+1}} \right|.[/tex]

If {a_n} is decreasing and lim a_n = 0, then what can we say about this limit?
 
Is the fact that [itex]a_n = 0[/itex] as n approaches infinity needed? I find no reason for it.
 
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