Image of Vector Subspace under Linear Transformation is Vector Subspace
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Theorem
Let $K$ be a field.
Let $X$ and $Y$ be vector spaces over $K$.
Let $U$ be a vector subspace of $X$.
Let $T : X \to Y$ be a linear transformation.
Then $T \sqbrk U$ is a vector subspace of $Y$.
Proof
Since $U$ is a non-empty set, we can apply the One-Step Vector Subspace Test to $T \sqbrk U$.
Let $\lambda \in K$ and $u, v \in T \sqbrk U$.
Then there exists $x, y \in U$ such that:
- $u = T x$
and:
- $v = T y$
Then:
\(\ds \lambda u + v\) | \(=\) | \(\ds \lambda T x + T y\) | ||||||||||||
\(\ds \) | \(=\) | \(\ds \map T {\lambda x + y}\) | Definition of Linear Transformation |
Since $U$ is a vector subspace, we have $\lambda x + y \in U$.
So $\map T {\lambda x + y} \in T \sqbrk U$.
That is, $\lambda u + v \in T \sqbrk U$.
So $T \sqbrk U$ is a vector subspace of $Y$.
$\blacksquare$