Under what conditions does there exist a linear function $L: \mathbb{R}^2 \rightarrow \mathbb{R}^2$ such that $L\left(\begin{array}{l}x_1 \\ y_1\end{array}\right)=\left(\begin{array}{l}a_1 \\ b_1\end{array}\right)$ and $L\left(\begin{array}{l}x_2 \\ y_2\end{array}\right)=\left(\begin{array}{l}a_2 \\ b_2\end{array}\right)$ ? Under what conditions is $L$ uniquely defined? In the latter case, write down the matrix representation of $L$.