Proving Continuity of Power Series Function

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Homework Help Overview

The discussion revolves around proving the continuity of a power series function, specifically f(x) = sum(a_n*x^n), within its radius of convergence. Participants are exploring the definition of continuity and how it applies to this context.

Discussion Character

  • Exploratory, Conceptual clarification, Assumption checking

Approaches and Questions Raised

  • The original poster attempts to establish continuity by using the definition, focusing on the relationship between |f(x) - f(a)| and |x - a|. Some participants question the validity of certain steps and suggest considering the difference quotient as a potential avenue for analysis.

Discussion Status

The discussion is ongoing, with participants providing hints and questioning assumptions about the prerequisites for understanding power series, particularly regarding differentiation. There is no explicit consensus, but several lines of reasoning are being explored.

Contextual Notes

Participants note a potential gap in prior knowledge, as differentiation has not yet been covered in their studies, which may impact the approach to proving continuity.

ricardianequiva
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Homework Statement


Show, from the definition of continuity, that the power series function f(x)=sum(a_n*x^n) is continuous for its radius of convergence.

Homework Equations


Definition of continuity

The Attempt at a Solution


Must show that for any |a| < R, given e>0 there exists d>0 such that |x-a|<d => |f(x) - f(a)|.
|f(x)-f(a)| < e.
|f(x) - f(a)| <= |f(x-a)|
Then I get stuck here.
Any help would be appreciated
 
Last edited:
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|f(x)-f(a)| is not less than |f(x-a)|. It's not like f is linear or something. |f(x)-f(a)|=|(f(x)-f(a))/(x-a)|*|x-a|. Now to get a d, you need a bound on |(f(x)-f(a))/(x-a)| near x=a. Hint: doesn't that look like a difference quotient?
 
Hmm we haven't done differentiation yet so I'm not sure how helpful the |(f(x)-f(a))/(x-a)| will be.
 
You are doing power series without having done differentiation!? That's an interesting pedagogical approach. You can still factor (x-a) algebraically from each power of f(x)-f(a), but I'm not sure how you show the rest of it converges without using the differentiability of power series.
 
yeah...
Thanks for the help though
 

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