The value of a Fourier series at a jump point (discontinuity)

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The discussion addresses the challenge of evaluating a Fourier series at a jump discontinuity, specifically at x=e, where the function has a value of f(e)=1 but approaches -infinity as x approaches e from the right. It highlights that the function does not meet the Dirichlet conditions due to this behavior, which complicates the application of standard convergence theorems. Participants emphasize the need to consider alternative approaches for handling such discontinuities in Fourier analysis. The conversation underscores the importance of understanding the implications of discontinuities on Fourier series convergence. This situation illustrates the complexities involved in analyzing functions with jump points in mathematical contexts.
Amaelle
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Homework Statement
calculate the value of the fourier serie at x=e for the e periodic function (0,e]
f(x)=log(x)
Relevant Equations
Fourier serie
Greetings
according to the function we can see that there is a jump at x=e and I know that the value of the function at x=e should be the average between the value of f(x) at this points
my problem is the following
the limit of f(x) at x=e is -infinity and f(e)=1
how can we deal with such situations?

thank you!
 
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Since ##f(0^+) = -\infty##, your function doesn't satisfy the Dirichlet conditions, so the usual theorem about convergence doesn't apply.
 
LCKurtz said:
Since ##f(0^+) = -\infty##, your function doesn't satisfy the Dirichlet conditions, so the usual theorem about convergence doesn't apply.
thanks a million!
 
First, I tried to show that ##f_n## converges uniformly on ##[0,2\pi]##, which is true since ##f_n \rightarrow 0## for ##n \rightarrow \infty## and ##\sigma_n=\mathrm{sup}\left| \frac{\sin\left(\frac{n^2}{n+\frac 15}x\right)}{n^{x^2-3x+3}} \right| \leq \frac{1}{|n^{x^2-3x+3}|} \leq \frac{1}{n^{\frac 34}}\rightarrow 0##. I can't use neither Leibnitz's test nor Abel's test. For Dirichlet's test I would need to show, that ##\sin\left(\frac{n^2}{n+\frac 15}x \right)## has partialy bounded sums...