9

This number is a composite.

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10*(2^2ⁿ + 1) + 9 gives primes for n = 1 to 7. Therefore, there are 7 known Fermat numbers which yields primes when a 9 is appended. [Wesolowski]

Submitted: 2009-05-15 19:30:29; Last Modified: 2011-02-06 19:44:26.