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triangle-man committed Mar 29, 2024
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22 changes: 12 additions & 10 deletions exercises/axler-3d.md
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Expand Up @@ -165,17 +165,19 @@ $U = \text{span }(\vec{f}_1, \dots, \vec{f}_n)$. First of all, it _is_
a subspace (since it’s the span of some vectors). The map $T|_U$ is
also surjective. For, given some $\vec{w}\in W$, then, since the
original $T$ was surjective, there must be $\vec{v}\in V$ with
$T(\vec{v}) = \vec{w}$. Now write $\vec{v}=\vec{u}+\vec{u}'$ where the
$\vec{u}$ is a combination of the $`\vec{e}`$s and the $\vec{u}'$ is a
combination of the $`\vec{f}`$s. Since $T(\vec{u})=\mathbf{0}$ we must
have $T(\vec{v}) = T(\vec{u}')$. And since $\vec{u}'\in U$, we have
shown that $T|_U$ is surjective.
$T(\vec{v}) = \vec{w}$. Now write $\vec{v}=\vec{u}+\vec{\nu}$ where
the $\vec{u}$ is a combination of the $`\vec{f}`$s and the $\vec{\nu}$
is a combination of the $`\vec{e}`$s (ie, in the null space). Since
$T(\vec{\nu})=\mathbf{0}$ we must have $\vec{w] = T(\vec{v}) =
T(\vec{u}) = T|_U(\vec{u}$. And since $\vec{u}\in U$, we have shown
that $T|_U$ is surjective.

Then, suppose $\vec{v}_1,\vec{v}_2\in U$ are distinct; it follows that
$T|_U(\vec{v}_1) \neq T|_U(\vec{v}_2)$, otherwise their difference
would be in $\text{null }T$, and hence that $T|_U$ is injective.

since $T|_U$ is surjective and injective, it is an isomorphism.
Finally, since $T|_U$ is surjective and injective, it is an
isomorphism.

## Question 18

Expand All @@ -190,18 +192,18 @@ $\mathcal{L}(\mathbf{F}, V)$.
What is $\mathcal{L}(\mathbf{F}, V)$? It is a rule which, given a
number in $\mathbf{F}$, produces a vector in $V$. Suppose
$\tilde{w}\in\mathcal{L}(\mathbf{F}, V)$ is one such rule. Then, for
any $\alpha\in\mathbf{F}$, $\tilde{w}(\alpha) = \tilde{w}(\alpha\times
1) = \alpha\tilde{w}(1)$. In other words, any
any $\alpha\in\mathbf{F}$, $`\tilde{w}(\alpha) =
\tilde{w}(\alpha\times 1) = \alpha\tilde{w}(1)`$. In other words, any
$\tilde{w}\in\mathcal{L}(\mathbf{F}, V)$ is defined entirely by its
action on $1$.

Consider the map, $\Omega\colon V\mathcal{L}(\mathbf{F}, V)$ which
Consider the map, $\Omega\colon V\to\mathcal{L}(\mathbf{F}, V)$ which
acts in the following way: for any $\vec{v}\in V$ set
$\Omega(\vec{v})$ to be the map

```math
\begin{aligned}
\Omega(\vec{v}) \colon \mathbf{F} &\to V \\
\Omega(\vec{v}) \colon \alpha \mapsto \alpha \vec{v}.
\end{algined}
\end{aligned}
```

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