Riemann-Lebesgue Lemma

Lemma
Let $f \in L^1$.

Then:
 * $\displaystyle \lim_{n \to \infty} \int f \left({x}\right)e^{i n x} \, \mathrm d x = 0$

That is, the Fourier transform of an $L^1$ function vanishes at infinity.

Proof
First suppose that $f \left({x}\right) = \chi_{\left({a, b}\right)} \left({x}\right)$, the characteristic function of an interval.

Then:
 * $\displaystyle \int f \left({x}\right)e^{i n x} \, \mathrm d x = \int_a^b e^{i n x} \, \mathrm d x = \frac {e^{i n b} - e^{i n a} } {i n} \to 0$ as $n \to \infty$

So the theorem is true for characteristic functions of intervals.

Now let $\displaystyle f = \sum_1^N c_n \chi_{\left({a_i, b_i}\right)} $, a simple function.

Then the same argument shows that the theorem holds for all such simple functions which are dense in $L^1$.

So let $\epsilon > 0$ and choose a simple function of the form above.

Call it $g_N \left({x}\right)$, such that $\displaystyle \int \left|{f - g_N}\right| < \epsilon$.

Now:
 * $\displaystyle \left|{\int f \left({x}\right)e^{i n x} \, \mathrm d x}\right| \le \int \left|{f \left({x}\right) - g_N \left({x}\right)}\right| \, \mathrm d x + \left|{\int g_N \left({x}\right) \, \mathrm d x}\right| \le \epsilon + \epsilon$

for sufficiently large $N$.

Since $\epsilon > 0$ is arbitrary, the proof is complete.