Let \(T:\mathbb{R}^4\rightarrow \mathbb{R}^3\) be a linear transformation.
Can \(T\) be an injection?
The answer is “No”.
We show this by establishing that \(\dim(\ker(T)) > 0.\)
Applying the rank-nullity theorem to \(T\), we have
\(\dim(\operatorname{range}(T)) + \dim(\ker(T)) = \dim(\mathbb{R}^4).\)
Hence,
\(\dim(\ker(T)) = \dim(\mathbb{R}^4) - \dim(\operatorname{range}(T)).\)
Note that \(\dim(\mathbb{R}^4) = 4\).
In addition,
\(\operatorname{range}(T)\) is a subspace of \(\mathbb{R}^3\) and therefore
has dimension at most \(3\). It follows that
\(\dim(\ker(T)) = 4 - \dim(\operatorname{range}(T)) \geq 4 - 3 =1.\)