11333...99997 (27-digits)

This number is a prime.

                                               1133355 5557777777 9999999997

The only prime formed by inserting in ascending order, between the digits of a double-digit number n, (n=17), the concatenation of the odd digits d, d times each of them, i.e., one 1, three 3’s, five 5’s, seven 7’s and nine 9’s. [Loungrides]