👉 In mathematics, an open set is a subset of Euclidean space that contains all its boundary points. It means that any point in the interior of the set can be reached from the boundary using straight-line distance. For example, consider the set S = {x ∈ R2 : x1 + 3x2 < 5}. This set is open because it contains all points (x,y) such that |x| + |y| ≤ 5. The set is not closed because
