77777...77777 (25-digits)

This number is a prime.

                                                 77777 7777774447 7777777777

The smallest "special" wing square-congruent prime of order 5. (Special wing primes are palindromic primes in base 10, and consisting of one central 3-digit number of identical digits surrounded by two wings having an equal amount of identical digits and different from the central digits). [Loungrides]