\[ \DeclareMathOperator{\Ext}{Ext} \]

Let \(X\) be a topological space and \(A\subseteq X\). The exterior of \(A\) is defined by

\begin{equation*} \Ext A = X\setminus \bar{A}, \end{equation*}

where \(\bar{A}\) is the closure of \(A\).

Remarks