Owlet puzzle #26
Released Friday, July 31, 2026 UTC
91
the answer for puzzle #26
The five clues
- A two-digit semiprime whose smallest prime factor is at least seven.
- Looks prime, but isn't.
- Its digit sum is ten — a two-digit number often mistaken for prime.
- Its factorization is 7 · 13.
- Also the smallest pseudoprime to base 3.
Backstory
91 = 7 · 13 is the classic trap of eye-primality: its factors are both prime and easy to miss. It's also the sum of two positive cubes (3³ + 4³ = 27 + 64 = 91), and equals T₁₃ = 1+2+…+13, making it a triangular semiprime.
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