MHB Fourier series, pointwise convergence, series computation

Markov2
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Let $f(x)=-x$ for $-l\le x\le l$ and $f(l)=l.$

a) Study the pointwise convergence of the Fourier series for $f.$
b) Compute the series $\displaystyle\sum_{n=0}^\infty \frac{(-1)^n}{(2n+1)}.$
c) Does the Fourier series of $f$ converge uniformly on $\mathbb R$ ?

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First I need to compute the Fourier series, so since $f$ is odd, then the Fourier series is just $\displaystyle\sum_{n=1}^\infty b_n\sin\frac{n\pi x}l$ where $b_n=\dfrac 2l\displaystyle\int_0^{l} f(x)\sin\frac{n\pi x}l\,dx$ so I'm getting $\displaystyle-\frac{x}{{{l}^{2}}}=\frac{1}{\pi }\sum\limits_{n=1}^{\infty }{\frac{{{(-1)}^{n}}}{n}\sin \frac{n\pi x}{l}},$ but now I don't know how to proceed with the pointwise convergence, also, how to do part b)?

Thanks for the help!
 
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Markov said:
Let $f(x)=-x$ for $-l\le x\le l$ and $f(l)=l.$

a) Study the pointwise convergence of the Fourier series for $f.$
b) Compute the series $\displaystyle\sum_{n=0}^\infty \frac{(-1)^n}{(2n+1)}.$
c) Does the Fourier series of $f$ converge uniformly on $\mathbb R$ ?

-------------

First I need to compute the Fourier series, so since $f$ is odd, then the Fourier series is just $\displaystyle\sum_{n=1}^\infty b_n\sin\frac{n\pi x}l$ where $b_n=\dfrac 2l\displaystyle\int_0^{l} f(x)\sin\frac{n\pi x}l\,dx$ so I'm getting $\displaystyle-\frac{x}{{{l}^{2}}}=\frac{1}{\pi }\sum\limits_{n=1}^{\infty }{\frac{{{(-1)}^{n}}}{n}\sin \frac{n\pi x}{l}},$ but now I don't know how to proceed with the pointwise convergence, also, how to do part b)?

Thanks for the help!

Hi Markov, :)

Firstly the definition of the function \(f\) seem to be erroneous, both \(f(x)=-x\mbox{ for }-l\leq x\leq l\) and \(f(l)=l\) cannot be true. So I shall neglect the latter part: \(f(l)=l\).

I think the Fourier series that you have obtained is also incorrect. It should be,

\[-x=\frac{2l}{\pi}\sum_{n=1}^{\infty}\frac{(-1)^{n}}{n}\sin\left(\frac{n\pi x}{l}\right)\]

Since both \(f(x)=-x\) and \(f'(x)=-1\) are continuous on \([-l,\,l]\) the Fourier series converges point-wise on the interval \((-l,\,l)\) (Refer Theorem 5.5 http://www.math.ucsb.edu/%7Egrigoryan/124B/lecs/lec5.pdf).

Substitute \(x=1\mbox{ and }l=2\) and we obtain,

\begin{eqnarray}

-\frac{\pi}{4}&=&\sum_{n=1}^{\infty}\frac{(-1)^{n}}{n}\sin\left(\frac{n\pi}{2} \right)\\

&=&\sum_{n=0}^{\infty}\frac{(-1)^{2n+1}}{2n+1}\sin\left(\frac{(2n+1)\pi}{2} \right)\\

&=&\sum_{n=0}^{\infty}\frac{(-1)^{2n+1}}{2n+1}(-1)^n\\

&=&\sum_{n=0}^{\infty}\frac{(-1)^{3n+1}}{2n+1}\\

\therefore\sum_{n=0}^{\infty}\frac{(-1)^{n}}{2n+1}&=&\frac{\pi}{4}

\end{eqnarray}

When \(x=l\) we have,

\[\left|f(l)-\sum_{n=1}^{N}\frac{(-1)^{n}}{n}\sin\left(\frac{n\pi l}{l}\right)\right|=l\]

Therefore by the definition of uniform convergence (Refer http://www.math.psu.edu/wade/M401-notes1.pdf) it is clear that the Fourier series of \(f\) is not uniformly convergent on \(\Re\).

Kind Regards,
Sudharaka.
 
Sudharaka said:
Hi Markov, :)

Firstly the definition of the function \(f\) seem to be erroneous, both \(f(x)=-x\mbox{ for }-l\leq x\leq l\) and \(f(l)=l\) cannot be true. So I shall neglect the latter part: \(f(l)=l\).

This is very likely to be intended to indicate the periodic extension: \(f(x)=f(x+2l)\) (or there is a mistake with the value at either \(+l\) or \(-l\), both end points would not normally be included in the domain for a Fourier Series).

Without the periodic extension the question of uniform convergence is moot, since the Fourier Series is periodic and so does not converge to the function outside of the interval.

CB
 
A sphere as topological manifold can be defined by gluing together the boundary of two disk. Basically one starts assigning each disk the subspace topology from ##\mathbb R^2## and then taking the quotient topology obtained by gluing their boundaries. Starting from the above definition of 2-sphere as topological manifold, shows that it is homeomorphic to the "embedded" sphere understood as subset of ##\mathbb R^3## in the subspace topology.

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