59

This number is a prime.

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59 is the only known mutidigit number n (up to 6000) such that sigma(n^n) + n is prime. The prime related to the next such number (if it exists) has more than 22500 digits. [Firoozbakht]

59 = 5*9 + π(5*9). It is probably the only multidigit number with this property. [Firoozbakht]