Can a discontinuous function have a uniformly convergent Fourier series?

[etf]

Let's say I have Fourier series of some function, f(t), f(t)=\frac{a0}{2}+\sum_{n=1}^{\infty}(an\cos{\frac{2n\pi t}{b-a}}+bn\sin{\frac{2n\pi t}{b-a}}), where a and b are lower and upper boundary of function, a0=\frac{2}{b-a}\int_{a}^{b}f(t)dt, an=\frac{2}{b-a}\int_{a}^{b}f(t)cos\frac{2n\pi t}{b-a}dt, bn=\frac{2}{b-a}\int_{a}^{b}f(t)sin\frac{2n\pi t}{b-a}dt. My question is, can I find derivative of Fourier series on this way:
\frac{d}{dt}\sum_{n=1}^{\infty}(an\cos{\frac{2n\pi t}{b-a}}+bn\sin{\frac{2n\pi t}{b-a}})=\frac{d}{dt}(\sum_{n=1}^{\infty}an\cos{\frac{2n\pi t}{b-a}}+\sum_{n=1}^{\infty}bn\sin{\frac{2n\pi t}{b-a}})=

 

[etf]

=\sum_{n=1}^{\infty}\frac{d}{dt}(an\cos{\frac{2n\pi t}{b-a}})+\sum_{n=1}^{\infty}\frac{d}{dt}(bn\sin{\frac{2n\pi t}{b-a}})
?
Are there some limitations when I can do this and when I can't do this?
PS. I had to write this in second message since for some reason my latex code can't fit in one message :)

 

[mathman]

I think I know what you meant - your equation is an identity (I don't think you meant it). I presume your question is can you differentiate term by term to get the derivative of a Fourier series. The answer is no different from the same question for any infinite series.

Google the question and you will get plenty of answers.

 

[HallsofIvy]

As long as a sum of functions is uniformly convergent, then you can differentiate the sum "term by term". Fourier series (and Taylor's series) are always uniformly convergent.

 

[The_Duck]

Fourier series (and Taylor's series) are always uniformly convergent.

I think the Fourier series of a discontinuous function is only pointwise convergent, not uniformly convergent.

 

[jbunniii]

I think the Fourier series of a discontinuous function is only pointwise convergent, not uniformly convergent.

Indeed, the partial sums of a Fourier series are continuous, and the uniform limit of a sequence of continuous functions is continuous. So no discontinuous function can have a uniformly convergent Fourier series.

Moreover, uniform convergence of a sequence of differentiable functions $f_n$ to some function $f$ is not sufficient to ensure that $f$ is differentiable. If the sequence of derivatives $f_n'$ converges uniformly, then it is sufficient, and $\lim f_n' = f'$.