Küpfmüller's uncertainty principle

Küpfmüller's uncertainty principle by Karl Küpfmüller in the year 1924 states that the relation of the rise time of a bandlimited signal to its bandwidth is a constant.^[1]

${\displaystyle \Delta f\Delta t\geq k}$

with ${\displaystyle k}$ either ${\displaystyle 1}$ or ${\displaystyle {\frac {1}{2}}}$