13715...15731 (64-digits)

This number is a prime.

      1371 5316312725 5511102320 4740958191 4095204710 2351125512 7633115731

Concatenation of the first 13 Mersenne numbers from 1 to 8191 and down to 1 again is a prime number. Note that 13 is an emirp. [Bajpai]