31

This number is a prime.

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© Baskin-Robbins

31 is the only prime P such that, for some n, P = rev(Sum_{i=0..pₙ} pₙ₊₁ⁱ) = Sum_{i=0..pₙ} pₙ₊₂ⁱ, where pₖ is the k-th prime. This occurs at n = 1: rev(3⁰ + 3¹ + 3²) = 5⁰ + 5¹ + 5² = 31. [Ahmed]