Progressive function

In mathematics a function f ∈ L²('R) is called progressive iff its Fourier transform is supported by positive frequencies only:

$\mathop {\rm {supp}} {\hat {f}}\subseteq \mathbb {R} _{+}$.

It is called regressive iff the time reversed function f(−t) is progressive, or equivalently, if

$\mathop {\rm {supp}} {\hat {f}}\subseteq \mathbb {R} _{-}$.

progressive function at PlanetMath.