13

This number is a prime.

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Photo credit: Patrick Kik

Pick a number n ≡ 2,4,7 (mod 9). Square n, then add the digits of the result; repeat. Eventually, you will reach 13. E.g., 2^2=04. 0+4=4. 4^2=16. 1+6=7. 7^2=49. 4+9=13. Also, 2+4+7=13. [Missailidis]