99999...99999 (53-digits)

This number is a prime.

                  999 9999999999 9999999999 9998999999 9999999999 9999999999

The first absolute largest palindromic prime. Absolute largest, i.e., formed from 9's and the middle digit 8, and, so, larger than any palindrome with the same number of digits. Palindromes of this pattern are expressed by the form 10^(2k+1)-10^k-1. [Silva]