@Acsor I don't know. [1,n] is sometimes used to represent the set of integers {1,2,3,..,n}. I don't know why they used the fancy brackets; I've never seen that notation before.
Hi, thanks for answering. I saw it once in a Logic class while studying Zermelo-Fraenkel axiomatic theory, but that was in a rather different context. I may as well just ask directly the OP
