Dilation of Subset of Vector Space Distributes over Sum/Finite Case
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Theorem
Let $K$ be a field.
Let $X$ be a vector space over $K$.
Let $A_1, \ldots, A_n \subseteq X$ and $\lambda \in \GF$.
Then:
- $\ds \lambda \sum_{j \mathop = 1}^n A_j = \sum_{j = 1}^n \paren {\lambda A_j}$
where:
- $\ds \lambda \paren \ldots$ denotes dilation by $\lambda$
- $\ds \sum_{j \mathop = 1}^n A_j$ denotes the linear combination of subsets of a vector space.
Proof
Let $x \in X$.
We have:
- $\ds x \in \lambda \sum_{j \mathop = 1}^n A_j$
if and only if there exists $x_j \in A_j$ for each $j \in \set {1, 2, \ldots, n}$ such that:
- $\ds x = \lambda \sum_{j \mathop = 1}^n x_j$
This is equivalent to:
- $\ds x = \sum_{j \mathop = 1}^n \lambda x_j \in \sum_{j = 1}^n \paren {\lambda A_j}$ for some $x_j \in A_j$.
Hence we have:
- $\ds x \in \lambda \sum_{j \mathop = 1}^n A_j$
- $\ds x \in \sum_{j = 1}^n \paren {\lambda A_j}$
Hence we obtain:
- $\ds \lambda \sum_{j \mathop = 1}^n A_j = \sum_{j = 1}^n \paren {\lambda A_j}$
$\blacksquare$