View source for Tightness of a topological space

{{TEX|done}}{{MSC|54A25}}

One of the [[cardinal characteristic]]s of a [[topological space]] $X$.  The ''local tightness'' $t(x,X)$ at a point $x \in X$ is the least cardinality $\mathfrak{t}\ge\aleph_0$ such that if $x$ is in the closure $\bar A$, then $A$ contains a subset $B$ of cardinality $\le \mathfrak{t}$ with $x \in\bar B$ .  The tightness $t(X)$ is the least upper bound of the local tightness.


====References====
* Mary Ellen Rudin, ''Lectures on Set Theoretic Topology'', American Mathematical Society (1975) {{ISBN|0-8218-1673-X}} {{ZBL|0318.54001}}