Image of Symmetric Set under Linear Transformation is Symmetric
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Theorem
Let $X$ and $Y$ be vector spaces over a subfield of $\C$.
Let $C$ be a symmetric subset of $X$.
Let $T : X \to Y$ be a linear transformation.
Then $\map T C$ is a symmetric subset of $Y$.
Proof
Let $y \in \map T C$.
Then there exists $x \in C$ such that $y = T x$.
Then from linearity we have $-y = \map T {-x}$.
Since $C$ is symmetric, we have $-x \in C$.
So $-y \in \map T C$.
So $\map T C$ is symmetric.
$\blacksquare$