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 A092506 Prime numbers of the form 2^n + 1. 41
 2, 3, 5, 17, 257, 65537 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 COMMENTS 2 together with the Fermat primes A019434. Obviously if 2^n + 1 is a prime then either n = 0 or n is a power of 2. - N. J. A. Sloane, Apr 07 2004 Numbers m > 1 such that 2^(m-2) divides (m-1)! and m divides (m-1)! + 1. - Thomas Ordowski, Nov 25 2014 From Jaroslav Krizek, Mar 06 2016: (Start) Also primes p such that sigma(p-1) = 2p - 3. Also primes of the form 2^n + 3*(-1)^n - 2 for n >= 0 because for odd n, 2^n - 5 is divisible by 3. Also primes of the form 2^n + 6*(-1)^n - 5 for n >= 0 because for odd n, 2^n - 11 is divisible by 3. Also primes of the form 2^n + 15*(-1)^n - 14 for n >= 0 because for odd n, 2^n - 29 is divisible by 3. (End) Exactly the set of primes p such that any number congruent to a primitive root (mod p) must have at least one prime divisor that is also congruent to a primitive root (mod p). See the links for a proof. - Rafay A. Ashary, Oct 13 2016 Conjecture: these are the only solutions to the equation A000010(x)+A000010(x-1)=floor((3x-2)/2). - Benoit Cloitre, Mar 02 2018 For n > 1, if 2^n + 1 divides 3^(2^(n-1)) + 1, then 2^n + 1 is a prime. - Jinyuan Wang, Oct 13 2018 LINKS Rafay A. Ashary, A Property of A092506 Eric Weisstein's World of Mathematics, Fermat Prime Eric Weisstein's World of Mathematics, Fermat Number MATHEMATICA Select[2^Range[0, 100]+1, PrimeQ] (* Harvey P. Dale, Aug 02 2015 *) PROG (PARI) print1(2); for(n=0, 9, if(ispseudoprime(t=2^2^n+1), print1(", "t))) \\ Charles R Greathouse IV, Aug 29 2016 (MAGMA) [2^n + 1 : n in [0..25] | IsPrime(2^n+1)]; // Vincenzo Librandi, Oct 14 2018 (GAP) Filtered(List([1..20], n->2^n+1), IsPrime); # Muniru A Asiru, Oct 25 2018 CROSSREFS A019434 is the main entry for these numbers. Sequence in context: A265426 A099936 A275584 * A275159 A127063 A127837 Adjacent sequences:  A092503 A092504 A092505 * A092507 A092508 A092509 KEYWORD nonn,hard AUTHOR Jorge Coveiro, Apr 05 2004 STATUS approved

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Last modified March 26 16:33 EDT 2019. Contains 321510 sequences. (Running on oeis4.)