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Homework Help: Prove function is continuous

  1. May 23, 2009 #1
    1. The problem statement, all variables and given/known data
    Prove that the function [tex] f(x) = \sum_{n=1}^{\infty} \frac{cos(n^2x)}{e^{nx^2}2^n} [/tex] is continuous on R.

    2. Relevant equations

    3. 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?
  2. jcsd
  3. May 23, 2009 #2
  4. May 23, 2009 #3


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    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.
  5. May 23, 2009 #4
    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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