# Sum of Summations equals Summation of Sum

## Theorem

Let $R: \Z \to \set {\T, \F}$ be a propositional function on the set of integers.

Let $\displaystyle \sum_{\map R i} x_i$ denote a summation over $R$.

Let the fiber of truth of $R$ be finite.

Then:

$\displaystyle \sum_{\map R i} \paren {b_i + c_i} = \sum_{\map R i} b_i + \sum_{\map R i} c_i$

### Infinite Sequence

Let $R: \Z \to \set {\mathrm T, \mathrm F}$ be a propositional function on the set of integers $\Z$.

Let $\displaystyle \sum_{\map R i} x_i$ denote a summation over $R$.

Let the fiber of truth of $R$ be infinite.

Let $\displaystyle \sum_{\map R i} b_i$ and $\displaystyle \sum_{\map R i} c_i$ be convergent.

Then:

$\displaystyle \sum_{\map R i} \paren {b_i + c_i} = \sum_{\map R i} b_i + \sum_{\map R i} c_i$

## Proof

Let $b_i =: a_{i 1}$ and $c_i =: a_{i 2}$.

Then:

 $\displaystyle \sum_{\map R i} \paren {b_i + c_i}$ $=$ $\displaystyle \sum_{\map R i} \paren {a_{i 1} + a_{i 2} }$ Definition of $b_i$ and $c_i$ $\displaystyle$ $=$ $\displaystyle \sum_{\map R i} \paren {\sum_{1 \mathop \le j \mathop \le 2} a_{i j} }$ Definition of Summation $\displaystyle$ $=$ $\displaystyle \sum_{1 \mathop \le j \mathop \le 2} \paren {\sum_{\map R i} a_{i j} }$ Exchange of Order of Summation $\displaystyle$ $=$ $\displaystyle \sum_{\map R i} a_{i 1} + \sum_{\map R i} a_{i 2}$ Definition of Summation $\displaystyle$ $=$ $\displaystyle \sum_{\map R i} b_i + \sum_{\map R i} c_i$ Definition of $b_i$ and $c_i$

$\blacksquare$