Intersection Condition for Direct Sum of Subspaces
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Theorem
Let $U$ and $W$ be subspaces of a vector space $V$.
Then $U + W$ is a direct sum if and only if $U \cap W = 0$.
Proof
We must first prove that if $U+W$ is a direct sum, then $U \cap W = 0$
Suppose $U + W$ is a direct sum. If a vector $\mathbf v \in U \cap W$, then $\mathbf 0 = \mathbf v + \paren {- \mathbf v}$, where $\mathbf v \in U$ and $- \mathbf v \in W$.
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