- 66 is an even composite number.
- 66 is the 11th triangular number.
- 66 is a repdigit number.
- If you flip 66 upside down, it becomes 99.
In Greek-based number-naming systems, 66 is associated with prefix "hexahexaconta-", and with prefix "sexasexaginti-" in Latin systems.
- OEIS, sequence A002808 (accessed 2020-11-13)
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