Is the Infinite Sum of Continuous Functions Also Continuous?

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Homework Help Overview

The discussion revolves around the continuity of the function defined by the infinite sum \( f(x) = \sum_{n=1}^{\infty} \frac{\cos(n^2x)}{e^{nx^2}2^n} \) on the real line. Participants are exploring the conditions under which the sum of continuous functions remains continuous.

Discussion Character

  • Conceptual clarification, Assumption checking

Approaches and Questions Raised

  • The original poster attempts to understand if demonstrating the differentiability of each term in the sum is sufficient for establishing the continuity of the entire function. Other participants suggest that uniform convergence is a necessary condition for the continuity of the sum, referencing specific convergence tests like the Weierstrass M-test.

Discussion Status

The discussion is ongoing, with some participants providing insights about uniform convergence and its relevance to the problem. The original poster expresses uncertainty about their current understanding of the necessary concepts, indicating a need for further exploration of the topic.

Contextual Notes

The original poster notes a lack of familiarity with series of functions and uniform convergence, which may limit their ability to fully engage with the problem at hand.

JG89
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Homework Statement


Prove that the function f(x) = \sum_{n=1}^{\infty} \frac{cos(n^2x)}{e^{nx^2}2^n} is continuous on R.


Homework Equations





The Attempt at a Solution




I haven't learned about a series of functions converging to a function yet, but would it be sufficient to show that each function in the infinite sum is differentiable, so each function in the infinite sum is continuous, meaning that the sum of the functions must also be continuous?
 
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JG89 said:

Homework Statement


Prove that the function f(x) = \sum_{n=1}^{\infty} \frac{cos(n^2x)}{e^{nx^2}2^n} is continuous on R.


Homework Equations





The Attempt at a Solution




I haven't learned about a series of functions converging to a function yet, but would it be sufficient to show that each function in the infinite sum is differentiable, so each function in the infinite sum is continuous, meaning that the sum of the functions must also be continuous?

No, it is not enough that each summand be continuous for the sum to be continuous. What one usually does in such types of question is show that the sequence of partial sums is uniformly convergent. There is a variety of tests out there for that purpose. One of the most useful is the so-called Weierstrass M-test and sure enough, it can be used on the series that interests you: http://en.wikipedia.org/wiki/Weierstrass_M-test.
 
Thanks for the replies guys.

I haven't learned about uniform convergence yet, so I guess I don't have the proper "machinery" to attack this problem.
 

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