This number is a prime.
777777755 5557555557 5555575555 5755555755 5557777777
The only “zeta prime” of order 7. (A zeta-prime is a palindromic prime of n^2 digits (n, odd > 2) arranged left-right, top-down, in an n-by-n array, where the first and the last row with the second diagonal make the letter Z and are consisting of identical digits a. The other digits of the array are also identical digits different from a. Thus, an order 7 zeta-prime is of the form aaaaaaabbbbbabbbbbabbbbbabbbbbabbbbbabbbbbaaaaaaaa.) [Loungrides]