Finding basis of a vector-subspace.

Question

How can I get to know a basis of a vector-subspace of $\mathbb{R}^{2 \times 2}$ formed by matrices $X$ that commute with the matrix: $$A=\left[\begin{matrix}0 & 1 \\ 0 & 0\end{matrix}\right]$$

Answer 1

Take an arbitrary matrix $X \in \mathbb{R}^{2\times 2}$ and compute the left and right products with $A$. This should give you restrictions on the coefficients in the matrix.

Say $$X=\left[\begin{matrix}x_{11} & x_{12} \\ x_{21} & x_{22} \end{matrix}\right] \; .$$

Then $XA=AX$ implies that $x_{21}=0$ and $x_{11}=x_{22}$. From this, it is straightforward to build a basis for the space of matrices $X$.