Fun:Fermat Primes

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Of the form 2^{2^{n}} + 1.

3, 5, 17, 257, 65537

There are no other known Fermat primes. It is believed that no others exist; if there are any more, they have at least two billion decimal digits.[1]


  1. Wilfrid Keller, Prime factors k*2^n + 1 of Fermat numbers Fm and complete factoring status