8
$\begingroup$

It is customary to teach Fourier transform on the real line by starting with functions from $L^1$, $L^2$ or the Schwartz space. It is not so easy to illustrate the theory by computing explicit pairs of transforms that are both elementary. The list given in the standard books is short: algebraic fractions, gaussian, exponentials, sinc and that's it.

I was surprised to learn that the Fourier transforms of $1/cosh(x)$ and $atan(x+1) - atan(x-1)$ can be computed explicitely and are of an elementary nature.

Are you aware of other $L^1$ or $L^2$ examples besides the classical ones?

I use the convention $\hat{f}(\xi) = \int_{-\infty}^\infty f(x) e^{-i\xi x} dx$, so that

$$\widehat{atan(x+1) - atan(x-1)} = 2\pi \ e^{-|x|} {\sin(x)\over x}$$

$$\widehat{1\over cosh(x)} = {\pi \over cosh({\pi x\over 2})}$$

EDIT1: I just noticed that $atan(x+1) - atan(x-1) = atan(2/x^2)$ and that the Fourier transform of this function is computed in the classical book of Oberhettinger.

Another explicit pair as suggested in the answer of C. Beenakker. $$ {\hbox{th}(x)\over x} = -2 \ln(\hbox{th}({\pi |\xi|\over 4})) $$

EDIT2: Nemo in this post gives many examples of Fourier transforms which are self-reciprocal and provides references. Some of them are elementary functions with explicit Fourier transforms. Note that the convention concerning the definition of the transform is different from the one used here.

$\endgroup$
  • 6
    $\begingroup$ Using convolutions and multiplication you'll get a whole bunch of more examples (e.g. transforms of B splines and truncated functions). $\endgroup$ – Dirk Mar 18 '17 at 20:00
  • 1
    $\begingroup$ And also linear combinations. This gives a lot (infinitely many) of interesting examples. $\endgroup$ – Alexandre Eremenko Mar 18 '17 at 20:34
  • 1
    $\begingroup$ For example any piecewise constant or piecewise linear function with compact support belongs to $L^1\cap L^2$ and has an elementary Fourier transform. $\endgroup$ – Alexandre Eremenko Mar 18 '17 at 20:42
9
$\begingroup$

The following example is interesting

$$\widehat{\sin(x)\over x \sqrt{|x|}} = \sqrt{2\pi} \ \left\{ { \matrix{ \scriptstyle \sqrt{|\xi+1|} + \sqrt{|\xi-1|} & \hbox{ if } |\xi| \leq 1, \cr {2\over \sqrt{|\xi+1|} + \sqrt{|\xi-1|}} & \hbox{ if } |\xi| \geq 1.\cr}} \right.$$

because it is in $L^1$ but not in $L^2$ and its Fourier transform is continuous but with infinite derivative at $\xi = \pm 1$.

The Fourier transform of $sin(x)\over x%{3/2}$

$\endgroup$
6
$\begingroup$

Does a Bessel function count as "elementary" ? The Fourier transform of $J_0(x)$ is $2/\sqrt{1-x^2}$ for $|x|<1$ and zero otherwise.

Concerning the combination of arctangents, you can Fourier transform ${\rm arctan}\,x$ itself into $-i\pi e^{-|x|}/x$.

Besides $1/\cosh x$, you can Fourier transform $x/\sinh x$ into $(\pi^2/2)/\cosh^2(\pi x/2)$, and $\tanh x$ into $-i\pi/\sinh(\pi x/2)$.

$$\widehat{1\over \cosh^2(x)} = {\pi \xi\over \sinh({\pi\xi\over 2})}$$

$\endgroup$
  • 3
    $\begingroup$ atan is not in $L^2$ nor in $L^1$. By elementary, I mean part of the undergraduate curriculum. tanh is nice. $\endgroup$ – coudy Mar 18 '17 at 17:31

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.