Confusion regarding notation/symbol? I was reading circular convolution topic from Digital Signal Processing Using Matlab  3rd Ed,by Proakis. I came across a strange term/symbol; I have highlighted it in attached snapshot
Update:Pg150; same chapter,same book(just before example 5.4) contains this notation and mentions that it is a rectangular window of length N
I have also attached snap of eq 5.24,in 2nd snap, where this RN(n) symbol appears also:
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.