Let $M$ be a matroid on a ground set $E$, and let $\operatorname{cl}_M$ denote its closure operator. For every subset $A \subset E$ and every element $e \in E$,
paragraph
admin
\begin{align*}
e \in \operatorname{cl}_M(A)
\end{align*}
latex_env
admin
if and only if either $e \in A$, or there exists a circuit $C$ of $M$ such that
paragraph
admin
\begin{align*}
e \in C \subset A \cup \{e\}.
\end{align*}