Similarity Mapping is Linear Operator

Theorem
Let $G$ be a vector space over a field $K$.

Let $\beta \in K$.

Then the mapping:
 * $s_\beta: G \to G$ defined by $s_\beta \left({\mathbf x}\right) = \beta \mathbf x$

is a linear operator on $G$.

If $\beta \ne 0$ then $s_\beta$ is an automorphism of $G$, and $\left({s_\beta}\right)^{-1} = s_{\beta^{-1}}$

The linear operators $s_\beta$, where $\beta \ne 0$, are called similarities of $G$.

Proof

 * Since $\beta \left({\mathbf x + \mathbf y}\right) = \beta \mathbf x + \beta \mathbf y$ and $\beta \left({\lambda \mathbf x}\right) = \lambda \left({\beta \mathbf x}\right)$, the fact of $s_\beta$ being a linear operator is immediately apparent.


 * We have $\left({s_{\beta^{-1}} \circ s_\beta}\right) \left({\mathbf x}\right) = \beta^{-1} \left({\beta \mathbf x}\right) = \mathbf x = \beta \left({\beta^{-1} \mathbf x}\right) = \left({s_\beta \circ s_{\beta^{-1}}}\right) \left({\mathbf x}\right)$

which proves the second bit.

Also known as
An older term for a similarity is similitude.