13

This number is a prime.

The smallest 2-digit number that produces the most primes by altering one digit of decimal expansion of n (without changing the number of digits). A set of eight primes can be produced in this manner {11, 17, 19, 23, 43, 53, 73, 83}. The sequence of such n-digit numbers starts 1, 13, 357, 4221, 42189, 344253, 1851759, 15848679, ... . [a(4)-a(8) by Metin Sariyar of Turkey]. See largest known. [Honaker]