53

This number is a prime.

The highest prime factor in a list of 48 semiprimes (by A. W. Johnson, 1978) the sum of whose reciprocals forms an Egyptian fraction of unity: (1/2*3 + 1/2*5 + 1/2*7 + 1/2*11 + 1/2*13 + 1/2*17 + 1/2*19 + 1/2*23 + 1/2*29 + 1/2*31 + 1/2*41 + 1/2*43) + (1/3*5 + 1/3*7 + 1/3*11 + 1/3*13 + 1/3*17 + 1/3*19 + 1/3*23 + 1/3*29 + 1/3*31 + 1/3*41) + (1/5*7 + 1/5*11 + 1/5*13 + 1/5*17 + 1/5*19 + 1/5*23 + 1/5*31 + 1/5*43 + 1/5*53) + (1/7*11 + 1/7*13 + 1/7*17 + 1/7*19 + 1/7*29 + 1/7*41) + (1/11*17 + 1/11*19 + 1/11*29) + (1/13*17 + 1/13*19 + 1/13*23 + 1/13*53) + (1/17*19 + 1/17*23 + 1/17*43 + 1/17*53) = 1. [Beedassy]