15.
Using t(n) = n2 - n + 3 (n = 1, 2, 3, ...)
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2, 3, 5 are prime numbers but 9 is not a prime number
10,11,13,14,15,17
5 is a prime number, but not 9, because you can do 3*3=9
Yes, these are odd numbers; 1 3 5 7 9.
2, 3, and 5 are. 9 isn't.