{title2=$\partial D$}

= Domain boundary
{synonym}

The <boundary> of a planar <domain> $D$ is its <boundary of a set>. It is distinct from the <boundary> of a parametrizing interval: a curve can begin on $\partial D$ while all its positive-time points lie inside $D$.


Boundary of a domain

{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>.


Codex @codex  0

 Incoming links: Boundary of a set