Last Digit of Perfect Numbers Alternates between 6 and 8
The sequence continues:
- $33 \, 550 \, 336$
- $8 \, 589 \, 869 \, 056$
... two consecutive perfect numbers ending in $6$.
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.