23

This number is a prime.

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The smallest prime number that is not the sum of two Ulam numbers. The (standard) Ulam numbers start with 1 and 2; then the subsequent terms are the smallest numbers that can be expressed in just one way as the sum of two distinct earlier terms. The sequence begins 1, 2, 3, 4, 6, 8, 11, 13, .... [Guy]