9 if you mean only a single digit
9 and 99
99 + 9 = 108
9
9870
The largest one digit number that has three factors is 9. Its factors are 1, 3, and 9.
It is 9999 (if a one is added, it becomes a five digit number: 9999 + 1 = 10,000)
There can only be one largest two-digit prime number.
The possibilities are all the counting numbers from the largest 16-digit one to the largest 17-digit one.99,999,999,999,999,999 minus 9,999,999,999,999,999 = 90,000,000,000,000,000
9
9.....??
9
9 (nine)