75555...55557 (31-digits)

This number is a prime.

                                          7 5555545555 5455555455 5554555557

The only 5-quintuplet of 5's prime. (A 5-quintuplet of 5's prime is a palindromic prime of form p=a55555b55555b55555b55555b55555a, where a, b, 5 distinct digits. [Loungrides]