Owlet puzzle #81
Released Thursday, September 24, 2026 UTC
2
the answer for puzzle #81
The five clues
- A single-digit positive integer.
- A prime number.
- The only even prime.
- Its factorial equals itself, and the sum of its digits is the number itself.
- The one integer for which n + n = n × n.
Backstory
The only even prime and the smallest of them all. It is also the base of binary and the unique prime p such that p + 1 is prime. Fermat's little theorem gives 2^{p−1} ≡ 1 (mod p) for every odd prime, making 2 the workhorse of primality testing.
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