# Are Fourier transforms unique?

1. Sep 28, 2010

### pellman

If

$$\int{g(x)e^{ikx}dx}=\int{h(x)e^{ikx}dx}$$

for all real k, can I conclude that g(x) = h(x) for all real x?

2. Sep 28, 2010

### disregardthat

By setting k = 0 you know they differ by a constant. Show that this constant is 0.

3. Sep 28, 2010

### Petr Mugver

You have to rely on Fourier inversion theorem, that you can find in any book that talks about Fourier transforms. In general, two functions (square integrable) that have the same Fourier transform are equal "almost everywhere", that is, everywhere except on a set of points with zero Lesbegue measure.

For k = 0 you only know they have the same average value, not that they differ by a constant.

4. Sep 28, 2010

### disregardthat

My apologies, you are right of course.

5. Sep 28, 2010

### mathman

Assuming the functions are nice, so that the inverse Fourier transform exists, then g(x)=f(x) almost everywhere. The expression for the inverse transform looks just like the expression for the transform except for a sign reversal in the exponent.

In other words, let H(t) be the Fourier transform of f(x) or g(x), then the inverse transform of H(t) (call it h(x)) equals f(x) (and g(x)) almost everywhere.

6. Sep 29, 2010

### pellman

Thanks! I will have to read up on Lesbegue measure.