Inverse of Linear Transformation is Linear Transformation
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Theorem
Let $K$ be a field.
Let $V$ and $U$ be vector spaces over $K$.
Let $A : V \to U$ be an invertible (in the sense of a mapping) linear transformation with inverse mapping $A^{-1} : U \to V$.
Then $A^{-1}$ is a linear transformation.
Proof
We aim to show that:
- $\map {A^{-1} } {\alpha x + \beta y} = \alpha A^{-1} x + \beta A^{-1} y$
for all $x, y \in U$ and $\alpha, \beta \in K$.
Since $A$ is a linear transformation, we have:
- $\map A {\alpha u + \beta v} = \alpha A u + \beta A v$
for each $u, v \in V$.
Note that we have $A^{-1} x \in V$ and $A^{-1} y \in V$, so that:
- $\map A {\alpha A^{-1} x + \beta A^{-1} y} = \alpha x + \beta y$
for each $x, y \in U$ and $\alpha, \beta \in K$.
Composing with $A^{-1}$, we then have:
- $\map {\paren {A^{-1} \circ A} } {\alpha A^{-1} x + \beta A^{-1} y} = \map {A^{-1} } {\alpha x + \beta y}$
From the definition of an inverse mapping, we therefore have:
- $\ds \map {A^{-1} } {\alpha x + \beta y} = \alpha A^{-1} x + \beta A^{-1} y$
for all $x, y \in U$ and $\alpha, \beta \in K$.
So:
- $A^{-1}$ is a linear transformation.
$\blacksquare$