Last Digit of Perfect Numbers Alternates between 6 and 8
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Conjecture
The last digit of the sequence of perfect numbers alternates between $6$ and $8$:
- $6$
- $28$
- $496$
- $8128$
Refutation
The sequence continues:
- $33 \, 550 \, 336$
- $8 \, 589 \, 869 \, 056$
... two consecutive perfect numbers ending in $6$.
$\blacksquare$
Also see
Historical Note
The conjecture that there is Last Digit of Perfect Numbers Alternates between $6$ and $8$ was made by Nicomachus of Gerasa in his Introduction to Arithmetic, published some time around the $2$nd century.
It was a simple extrapolation from the knowledge of the perfect numbers at the time.
Some sources suggest that Iamblichus Chalcidensis made these conjectures, but this appears to be incorrect.
Sources
- 1986: David Wells: Curious and Interesting Numbers ... (previous) ... (next): $28$
- 1997: David Wells: Curious and Interesting Numbers (2nd ed.) ... (previous) ... (next): $28$