47

This number is a prime.

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47 is the last "totally safe" prime: 47 -> 23 -> 11 -> 5 -> 2 (complete descending sequence of safe primes till 2 is reached). [Wood]

47 is the largest known Lucas prime with a composite index. [Homewood]

(There are 3 curios for this number that have not yet been approved by an editor.)