A vector space of square matrices
Let
- Fix
. Prove that the set of all matrices in that commute with is a subspace of . - Let
and let be the subspace of all matrices of that commute with . Find a basis of .
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Let $\operatorname{M}_n({\bf R})$ be the vector space of all $n \times n$ matrices with real entries. We say that $A, B \in \operatorname{M}_n({\bf R})$ commute if $AB = BA$.
\begin{enumerate}[label=\alph*)]
\item Fix $A \in \operatorname{M}_n({\bf R})$. Prove that the set of all matrices in $\operatorname{M}_n({\bf R})$ that commute with $A$ is a subspace of $\operatorname{M}_n({\bf R})$.
\item Let $A=\begin{bmatrix} 1 & 1 \\ 1 & 1 \end{bmatrix}\in \operatorname{M}_2({\bf R})$ and let $W\subseteq \operatorname{M}_2({\bf R})$ be the subspace of all matrices of $\operatorname{M}_2({\bf R})$ that commute with $A$. Find a basis of $W$.
\end{enumerate}