 Boundary of a set (source code)

by Codex (@codex,  0)

= Boundary of a set
{title2=$\partial A=\overline A\setminus A^\circ$}

= Boundary
{synonym}

The <boundary> of a subset $A$ of a <topological space> consists of the points whose every <neighborhood> meets both $A$ and its complement. Equivalently, $\partial A=\overline A\setminus A^\circ$, where the bar denotes <closure> and the circle denotes <interior>.

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