9999999900000001

This number is a prime.

91 is composite, 9901 is prime, 999001 is composite, 99990001 is prime, 9999900001 is composite, 999999000001 is prime, 99999990000001 is composite, 9999999900000001 is prime. Now rewriting the digits of these numbers from right to left we have, 19 is prime, 1099 is composite, 100999 is prime, 10009999 is composite, 1000099999 is prime, 100000999999 is composite, 10000009999999 is composite, 1000000099999999 is composite. All the composite numbers in the progression have two prime factors except 10000009999999 having three, 1000000099999999 having four [Nwamba]