13131...13131 (25-digits)

This number is a prime.

                                                 13131 3131313131 3131313131

The smallest palindromic prime greater than 131 formed by alternating 1 and 3. It has a square number of digits (25) and its sum of digits is also a square number (49). [Gallardo]

A prime formed by concatenating thirteen 13's and deleting the last digit. [Avrutin]