Let $G = (V,E)$ be a graph with $V$ set of vertices, $E$ edges. What is the set of mutual neigbors of a set $R \subset V(G)$? In particular, what is a mutual neighbor of a set $R \subset V(G)$?
This notation is at the end of page 2 of the following paper and I could not find any definition on the Internet: https://www.ethz.ch/content/dam/ethz/special-interest/eth-its/Documents/lecture5.pdf
Thank you in advance!
Just based on experience, I would guess the mutual neighbors of $R$ are the vertices adjacent to all the vertices of $R$.
Some evidence to back it up (google books).