11111...11111 (25-digits)

This number is a prime.

                                                 11111 1888118881 1888111111

This congruent prime of order-5 of form aaaaaabbbaabbbaabbbaaaaaa is the first known “perimeter prime”, i.e., a prime formed if we put all its digits in a 5 x 5 array then the digits that are in the perimeter of the square are equal to a and all the other are equal to b. Note that there is no other such prime of order 7, 9 or 11. [Loungrides]