Image of Convex Set under Linear Transformation is Convex

Theorem
Let $X$ and $Y$ be vector spaces over a subfield of $\C$.

Let $C \subseteq X$ be convex.

Let $T : X \to Y$ be a linear transformation.

Then $\map T C \subseteq Y$ is convex.

Proof
Let $y_1, y_2 \in \map T C$ and $\lambda \in \closedint 0 1$.

Then, there exists $x_1, x_2 \in C$ such that:


 * $y_1 = T x_1$

and:


 * $y_2 = T x_2$

Then, we have:


 * $\lambda y_1 + \paren {1 - \lambda} y_2 = \lambda T x_1 + \paren {1 - \lambda} T x_2 = \map T {\lambda x_1 + \paren {1 - \lambda} x_2}$

Since $C$ is convex, we have:


 * $\lambda x_1 + \paren {1 - \lambda} x_2 \in C$

So, we have:


 * $\map T {\lambda x_1 + \paren {1 - \lambda} x_2} \in \map T C$

That is:


 * $\lambda y_1 + \paren {1 - \lambda} y_2 \in \map T C$

So $\map T C$ is convex.