One-Step Vector Subspace Test

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

Let $U \subseteq V$ be a non-empty subset of $V$ such that:


 * $\forall u, v \in U: \forall \lambda \in K: u + \lambda v \in U$

Then $U$ is a subspace of $V$.

Corollary
Let $U \subseteq V$ be a non-empty subset of $V$ such that:


 * $(1): \qquad \forall u \in U, \lambda \in K: \lambda u \in U$
 * $(2): \qquad \forall u, v \in U: u + v \in U$

Then $U$ is a subspace of $V$.