73

This number is a prime.

73 is the only Sheldon prime, i.e., i) whose binary representation is palindromic (1001001₂) and ii) which belongs to an emirp pair (P_n, P_m) such that subscripts (m, n) = (21, 12) are also reversals of each other and n has a prime decomposition 21 = 3*7 that concatenates back to P_m (Carl Pomerance, Chris Spicer, February 2019). [Beedassy]