👉 In mathematics, a semi-open set is a subset of a topological space that is neither open nor closed. It means that it contains an open and closed neighbor, but not both. For example: - The set {x ∈ R | x < 0} is semi-open because it contains the open interval (−∞, 0), which is not closed. - The set of all real numbers, denoted as R, is semi-open because it contains the entire real line, but
