👉 In mathematics, a non-closed set is one that contains two points where the closure of the set does not equal the boundary. This means that if we have a closed set and an open set, then the union of the two sets will not be empty or a proper subset of the original closed set. For example: - The set of all real numbers x such that x < 0 is non-closed because it contains -1 as a point where the closure does not equal the boundary.
