Confusion regarding notation/symbol?
This symbol denotes a rectangular pulse of length $N$:
$$\mathcal{R}_N(n)=\begin{cases}1,&amp;0\le n\le N-1\\0,&amp;\textrm{otherwise}\end{cases}$$
I'm not sure where it is defined for the first time, but this definition is clear from the equation above Eq. $(5.24)$ on page $130$ of the 3rd edition of Digital Signal Processing Using Matlab by V.K. Ingle and J.G. Proakis.