# Partition

2010 Mathematics Subject Classification: Primary: 54B [MSN][ZBL]

## Of a topological space

A closed set $E$ in a topological space $X$ that partitions $X$ between two given sets $P$ and $Q$ (or, in other words, separates $P$ and $Q$ in $X$), i.e. such that $X \setminus E = H_1 \cup H_2$, where $H_1$ and $H_2$ are disjoint and open in $X \setminus E$, $P \subseteq H_1$, $Q \subseteq H_2$ ($P$ and $Q$ are open in all of $X$). A partition is called fine if its interior is empty. Any binary decomposition (i.e. a partition consisting of two elements) $\alpha = (A_1,A_2)$ of a space $X$ defines a fine partition in $X$: $B$ is the boundary of $A_1$, which is the boundary of $A_2$, where $X\setminus B = O_1 \cup O_2$, in which $O_i$ is the interior of $A_i$, $i=1,2$. The converse is also true. In essence, the concept of a partition between sets leads to the concept of connectedness. The converse also applies: A space $X$ is disconnected if $\emptyset$ is a partition between non-empty sets.

If $A$ and $B$ are disjoint subsets of a space $X$, then a separator between$A$ and $B$ is a set $S$ such that $X \setminus S = V \cup W$ with $V$ and $W$ disjoint and open in $X \setminus S$, and $A \subseteq V$ and $B \subseteq W$. So a partition is a closed separator.
A set $C$ is a cut between $A$ and $B$ if $C$ intersects every continuum that intersects both $A$ and $B$.
One readily sees that every partition is a separator and that every separator is a cut, and the following examples show that the notions are in general distinct: the open interval $(0,1)$ is a separator between $\{0\}$ and $\{1\}$ in the interval $[0,1]$, but not a partition; in the well-known subspace $\{0\} \times [-1,1] \cup \{ (x,\sin(1/x)) : 0 < x \le 1 \}$ of the Euclidean space, the point $(0,0)$ is a cut but not a separator between the points $(0,1)$ and $(1,\sin 1)$.