# Amicable Pair/Examples/17,296-18,416

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## Example of Amicable Pair

$17 \, 296$ and $18 \, 416$ are the $8$th amicable pair:

- $\map {\sigma_1} {17 \, 296} = \map {\sigma_1} {18 \, 416} = 35 \, 712 = 17 \, 296 + 18 \, 416$

## Proof

By definition, $m$ and $n$ form an amicable pair if and only if:

- $\map {\sigma_1} m = \map {\sigma_1} n = m + n$

where $\sigma_1$ denotes the divisor sum function.

Thus:

\(\ds \map {\sigma_1} {17 \, 296}\) | \(=\) | \(\ds 35 \, 712\) | $\sigma_1$ of $17 \, 296$ | |||||||||||

\(\ds \) | \(=\) | \(\ds 17 \, 296 + 18 \, 416\) | ||||||||||||

\(\ds \) | \(=\) | \(\ds \map {\sigma_1} {18 \, 416}\) | $\sigma_1$ of $18 \, 416$ |

$\blacksquare$

## Historical Note

The amicable pair $17 \, 296$ and $18 \, 416$ were discovered by Pierre de Fermat in $1636$.

As such, it appears that he re-discovered what had previously been discovered by the medieval Arab school.

It was, however, the second amicable pair to be known of by the Western mathematical world after $220$ and $284$, known of old.

## Sources

- 1986: David Wells:
*Curious and Interesting Numbers*... (previous) ... (next): $17,296$ - 1997: David Wells:
*Curious and Interesting Numbers*(2nd ed.) ... (previous) ... (next): $17,296$

- Weisstein, Eric W. "Amicable Pair." From
*MathWorld*--A Wolfram Web Resource. http://mathworld.wolfram.com/AmicablePair.html