Explain why the translation function $T: \mathbb{R}^2 \rightarrow \mathbb{R}^2$, defined by $T\left(\begin{array}{l}x \\ y\end{array}\right)=\left(\begin{array}{l}x+a \\ y+b\end{array}\right)$ for $\quad a, b \in \mathbb{R}$, is almost never linear. Precisely when is it linear? $a, b \in \mathbb{R}$, is almost never linear. Precisely when is it linear?