Solution
= Solution
If $x\in Y$, then $Y+x=Y$, so $r(Y+x)=r(Y)$. By the definition of the <closure of a matroid>, $x\in\sigma(Y)$. Hence \b[$Y\subseteq\sigma(Y)$].
= Solution
If $x\in Y$, then $Y+x=Y$, so $r(Y+x)=r(Y)$. By the definition of the <closure of a matroid>, $x\in\sigma(Y)$. Hence \b[$Y\subseteq\sigma(Y)$].