\begin{definition}[Definition:Linear Group Action]
Let $\left({V, +, \cdot}\right)$ be a vector space over a field $\left({k, \oplus, \circ}\right)$.
Let $G$ be a group.
Let $\phi : G \times V \to V$ be an action of $G$ on $V$.
Then $\phi$ is a '''(left) linear group action''' {{iff}} it is compatible with the linear structure of $V$ in the following sense:
: $(1): \quad \forall v_1, v_2 \in V: g \in G: \phi \left({g, v_1 + v_2}\right) = \phi \left({g, v_1}\right) + \phi \left({g, v_2}\right)$
: $(2): \quad \forall \lambda \in k, g \in G, v \in V: \phi \left({g, \lambda \cdot v}\right) = \lambda \cdot \phi \left({g, v}\right)$
\end{definition}