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Old 2020-10-14, 12:37   #7
henryzz
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"David"
Sep 2007
Cambridge (GMT/BST)

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Quote:
Originally Posted by bur View Post
Not really related to this specific question, but also a question about 2^n + 1:


Why aren't they researched in regard to being prime? It is basically a Proth number with k = 1. I just checked for n <= 200 and got primes only for n = 1, 2, 4, 8, 16 - or F(0) to F(4).

Is this sequence similar to Fermat and there are no known primes for 2^n + 1 other than those I mentioned above? If so, is it proven?

Googling didn't yield any results surprisingly, but looking up formulas can be hard.
For 2^n+1 to be prime n must be a power of 2 https://mathworld.wolfram.com/FermatNumber.html has a proof of this.
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