A type of [[Finite Impulse Response Filters|signal filter]]. $\huge \begin{align} y_{t}=x_{t} - R^{L}x_{t-L} \end{align} $ Where $L\in \Z^{+}$ delay and $R\in\R$. Usually, $R \approx 1, R<1$ (close to). The [[Transfer Function]] is: $\huge \begin{align} \mathcal{H}(z) &= 1- R^{L}z^{-L} \\ &= \frac{z^{L}-R^{L}}{z^{L}} \end{align} $ The roots of $\mathcal{H}$ are: $\huge \begin{align} z &= R e^{i \frac{2\pi}{L} k}, k \in \Z^{+} \end{align} $ Which are $L$ complex numbers on the circle of radius $R$. $\huge \begin{align} |H(\omega)| &= \frac{{\left|e^{i\omega L} -R^{L}\right|}}{|e^{i\omega L}|} \\ &= \left| e^{i \omega L} - R^{L}\right| \\ &= \prod_{k=0}^{L-1} \left| e^{i\omega} -\mathrm{Re}^{i \frac{2\pi}{L}k}\right| \end{align} $ Which is the the product of the distances of $z=e^{i\omega}$ to all roots of $z^{L}-R$ (evenly spaced numbers on the circle of radius $R$).