open set - PlanetMath
... A set containing x is called a neighborhood of x if there is an open ball around x which is a subset of the neighborhood. These neighborhoods have some properties, which can be used to define a topological space using the Hausdorff axioms for neighborhoods, by which again an open set within a topolo ...
... A set containing x is called a neighborhood of x if there is an open ball around x which is a subset of the neighborhood. These neighborhoods have some properties, which can be used to define a topological space using the Hausdorff axioms for neighborhoods, by which again an open set within a topolo ...
MA651 Topology. Lecture 3. Topological spaces.
... Definition 20.1. Let X be a set. A topology (or topological structure) in X is a family T of subsets of X that satisfies: 1. Each union of members of T is also a member of T . 2. Each finite intersection of members of T is also a member of T . 3. Ø and X are members of T . Definition 20.2. A couple ...
... Definition 20.1. Let X be a set. A topology (or topological structure) in X is a family T of subsets of X that satisfies: 1. Each union of members of T is also a member of T . 2. Each finite intersection of members of T is also a member of T . 3. Ø and X are members of T . Definition 20.2. A couple ...