# Fourier series of functions with points of discontinuity

1. Nov 17, 2012

### jorgdv

If you have a function with countable discontinuities on an interval, I know that the Fourier series will converge to that function without those discontinuities. But how could you explain that formally? If the basis of the fourier series span the space L^2[a,b], that would include functions with countable point-discontinuities, right?

2. Nov 17, 2012

### mathman

The Fourier series for L^2 functions will converge to the function at all points of continuity and will converge to the average value at the discontinuities.

3. Nov 17, 2012

### micromass

Staff Emeritus
Not really. There is a continuous function whose Fourier series does not converge. What you say is only true with some additional conditions, for example a Lipschitz condition or a differentiable condition.

4. Nov 17, 2012

### mathman

It depends on what you mean by convergence. I was talking about convergence in the L^2 norm.

5. Nov 17, 2012

### micromass

Staff Emeritus
Then it's still wrong. The Fourier series of a function always converges to the function in the $L^2$-norm. Doesn't matter what the discontinuities are.

http://en.wikipedia.org/wiki/Riesz–Fischer_theorem

6. Nov 18, 2012

### jorgdv

What I was thinking is that in the L2 space there is an equivalence relation such that if the Lebesgue integral of the diference is 0, then they are equivalent. However, the functions in the trigonometric basis of Fourier are contained in C[a,b], and because C[a,b] is closed under addition, the infinite linear combination with real coefficients will also be contained in C[a,b]. So the Fourier series will converge to the continuous equivalent function in the L2 space. Is that right?

7. Nov 18, 2012

### mathman

8. Nov 18, 2012

### micromass

Staff Emeritus
What do you mean with "infinite linear combination" and what do you mean with "converge". The answers to your question depend on that. There are multiple ways to interpret convergence or summation of functions.

9. Nov 18, 2012

### jorgdv

With "infinite linear combination" I mean an infinite sum of elements contained in the space, in this case, scaled by real numbers (each element). And with "converge" in that context I meant pointwise.

10. Nov 19, 2012

### mathman

Not true: C[a,b] is not closed under infinite addition. All truncated Fourier series are continuous!

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