Editing Sign function (section)

=== Fourier transform ===
The [[Fourier transform]] of the signum function is<ref>{{cite journal|last1=Burrows|first1=B. L.|last2=Colwell|first2=D. J.|title=The Fourier transform of the unit step function|journal=International Journal of Mathematical Education in Science and Technology|date=1990|volume=21|issue=4|pages=629–635|doi=10.1080/0020739900210418}}</ref>
<math display="block">PV\int_{-\infty}^\infty (\sgn x) e^{-ikx}\text{d}x = \frac{2}{ik} \qquad \text{for } k \ne 0,</math>
where <math>PV</math> means taking the [[Cauchy principal value]].