47

This number is a prime.

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The only double-digit prime from which no one prime is obtained by inserting between its digits d d's (where d is any digit). [Loungrides]

The largest double-digit prime ab such that a^a + b^b is prime. [Loungrides]

(47, 53, 59, 61) is the only tetrad of successive double-digit primes p < q < r < s such that (p+q) divides r*s+1, i.e., (47+53) divides 59*61+1. [Loungrides]

The smaller of only two double-digit primes, whose cube, (103823), can be represented as the concatenation of two 3-digit primes. [Loungrides]