Permutation of Indices of Summation/Proof
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Theorem
- $\ds \sum_{\map R j} a_j = \sum_{\map R {\map \pi j} } a_{\map \pi j}$
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 |
$\blacksquare$
Sources
- 1997: Donald E. Knuth: The Art of Computer Programming: Volume 1: Fundamental Algorithms (3rd ed.) ... (previous) ... (next): $\S 1.2.3$: Sums and Products: $(18)$