👉 In mathematics, a semi-open set is a subset of a topological space that is both open and closed. This means that it is both an open set and a closed set. An example of a semi-open set is the real line R with its usual topology, which is given by the open sets as follows: - The empty set - The singletons {0}, {1}, ..., {n} for n>0 - The whole set R This means that every point x
