19

This number is a prime.

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The only prime number p such that sod(p*reversal(p)) is p itself, where sod is sum of digits. [Kholdi]

19 is the smallest prime such that there exists a multiple having the same sum of digits as its digital root, e.g., 75 = 19 x 3; 7 + 5 = 12, 1 + 2 = 3, and 3 = 3. [Kholdi]