61

This number is a prime.

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61 is the smallest number m and only two-digit number m such that the reversal of concatenation of the first m natural numbers is prime. [Firoozbakht]

61!*16! - 1 is prime with prime sum of digits. [Firoozbakht]

61 is the 7th prime of the form 3k+1 and 61 = (77+777)/(7+7). Note that seven 7's are used. [Firoozbakht]