progressive function

Definition

A function $f\in L^2(ℝ)$ is called progressive iff its Fourier transform is supported by positive frequencies only:

$$supp\widehat{f}\subseteq {ℝ}_{+}.$$

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

$$supp\widehat{f}\subseteq {ℝ}_{-}.$$