### Author Topic: problem 2  (Read 4058 times)

#### Djirar

• Full Member
•   • Posts: 24
• Karma: 8 ##### problem 2
« on: December 20, 2012, 01:30:26 PM »
Consider a $2\pi$-periodic function $f$ with full Fourier series
$$\sum_{n \in \mathbb{Z}} c_n e^{i n x}.$$
Suppose that the Fourier coefficients decay fast enough to satisfy
$$\sum_{n \in \mathbb{Z}} |n| \cdot |c_n| < 17.$$
Prove that $f'$ is bounded.
« Last Edit: December 20, 2012, 01:48:13 PM by Victor Ivrii »

#### Chen Ge Qu

• Full Member
•   • Posts: 16
• Karma: 8 ##### Re: problem 2
« Reply #1 on: December 20, 2012, 01:35:34 PM »
I thought we were supposed to wait until Prof. Ivrii posted the problems...?

In any case, my solution to Problem 2 is attached.

#### Pei Zhou

• Newbie
• • Posts: 4
• Karma: 3 ##### Re: problem 2
« Reply #2 on: December 20, 2012, 04:49:39 PM »

#### Victor Ivrii ##### Re: problem 2
« Reply #3 on: December 22, 2012, 01:02:24 PM »
I will leave this problem to grade to Prof. Colliander. None of the posted solutions satisfies me.

Proof. Consider
\begin{equation}
g(x)=\sum_{n=-\infty}^\infty nc_n e^{inx}.
\label{eq-1}
\end{equation}
Since $\sum_{n=-\infty}^\infty |nc_n e^{inx}|=\sum_{n=-\infty}^\infty |n|\cdot |c_n|\le M$, series (\ref{eq-1}) converges uniformly and therefore one can integrate it termwise:
\begin{equation}
\int_0^x g(x)\,dx=\sum_{n=-\infty}^\infty \int_0^x nc_n e^{inx}\,dx =\sum_{n=-\infty}^\infty \int_0^x c_n \bigl(e^{inx}-1)=f(x)-f(0)
\label{eq-2}
\end{equation}
Therefore $f(x)$ is differentiable and $f'(x)=g(x)$.

PS. You can differentiate series termwise if you get uniformly converging series. Nobody mentioned this.