A hierarchy of regularity conditions on a topological space X: T0 (distinguishability), T1 (points closed), T2 (Hausdorff), T3 (regular + T1), T4 (normal + T1). Each implies the lower; Hausdorff and normal are the most commonly used.
A hierarchy of regularity conditions on a topological space X: T0 (distinguishability), T1 (points closed), T2 (Hausdorff), T3 (regular + T1), T4 (normal + T1). Each implies the lower; Hausdorff and normal are the most commonly used.