# Permutation of Indices of Summation

## Theorem

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

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

Then:

$\ds \sum_{\map R j} a_j = \sum_{\map R {\map \pi j} } a_{\map \pi j}$

where:

$\ds \sum_{\map R j} a_j$ denotes the summation over $a_j$ for all $j$ that satisfy the propositional function $\map R j$
$\pi$ is a permutation on the fiber of truth of $R$.

### Infinite Series

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

Let $\ds \sum_{\map R i} a_i$ be absolutely convergent.

Then:

$\ds \sum_{\map R j} a_j = \sum_{\map R {\map \pi j} } a_{\map \pi j}$

## Proof

 $\ds \sum_{\map R {\map \pi j} } a_{\map \pi j}$ $=$ $\ds \sum_{j \mathop \in \Z} a_{\map \pi j} \sqbrk {\map R {\map \pi j} }$ Definition of Summation by Iverson's Convention $\ds$ $=$ $\ds \sum_{j \mathop \in \Z} \sum_{i \mathop \in \Z} a_i \sqbrk {\map R i} \sqbrk {i = \map \pi j}$ $\ds$ $=$ $\ds \sum_{i \mathop \in \Z} a_i \sqbrk {\map R i} \sum_{j \mathop \in \Z} \sqbrk {i = \map \pi j}$ $\ds$ $=$ $\ds \sum_{i \mathop \in \Z} a_i \sqbrk {\map R i}$ $\ds$ $=$ $\ds \sum_{\map R i} a_i$ $\ds$ $=$ $\ds \sum_{\map R j} a_j$ Change of Index Variable of Summation

$\blacksquare$

## Also known as

The operation of permutation of indices of a summation can be seen referred to as a permutation of the range.

However, as the term range is ambiguous in the literature, and as its use here is not strictly accurate (it is the fiber of truth of $R$, not its range, which is being permuted, its use on $\mathsf{Pr} \infty \mathsf{fWiki}$ is discouraged.