17902...20971 (8271-digits)

This number is a probable-prime (likely to be a prime but we have not (re-)proven it on this site).

The bottom row of a palindromic prime pyramid (starting with 2) of step size 5 and height 828, where each palindromic prime p is the smallest p that contains p in the row above. It is the p associated with a(5) in the following sequence: 3, 11, 26, 135, 828, ... . [Gupta]