# Open subset in Irreducible Topological Space is dense.

Show that every non-empty open subset of an irreducible topological space is dense.

I know a lemma that states that $U \subset$X is dense iff for all $A \in \tau$, $A \cap U \neq \emptyset$.

So then let U be an open set in $(X, \tau_{zar})$ that is irreducible. Then I want to show that for all $A \in \tau$, $A \cap U \neq \emptyset$. I don't know how to show this though, nor how the irreducibility fits in.

• You mean "... iff for all $A\in \tau$ with $A\ne\emptyset$, $A\cap U\ne \emptyset$" (and in fact this is hardly a lemma, but a possible definition of dense. – Hagen von Eitzen Mar 30 '14 at 10:22

Let $U$ be a nonempty open subset of an irreducible topological space $X$. Denote by $\overline{U}$ the closure of $U$ in $X$. Then $(X - U, \overline{U})$ is a decomposition of $X$. Because $X$ is irreducible, one of these sets equals $X$. Since $U$ is nonempty,$$X \setminus U \neq U \implies \overline{U} = X.$$
• Should it be $X\setminus U\neq X$? – math112358 Jun 9 at 23:10