28608...23551 (27-digits)

This number is a prime.

                                               2860817 5565529146 4259223551

The largest known prime factor in the 12th term of the Lucas-Lehmer sequence, i.e., (((((((((((4^2-2)^2-2)^2-2)^2-2)^2-2)^2-2)^2-2)^2-2)^2-2)^2-2)^2-2)^2-2, as of December 16, 2020. [Honaker]