OFFSET
1,1
COMMENTS
The old definition was: Numbers n such that A123669(n) = -1, or no generalized Fermat prime exists of the form (2n)^(2^k) + 1. But that sequence is probably missing a lot of terms, such as {6, 9, 11, 19, 21, 25, 26, 29, 30, 31, ...}, where no generalized Fermat prime has been found yet, and it seems unlikely any exist. Currently it can only be proved that none exist if n is of form b^m/2 for even b and odd m > 1. The listed terms are the first numbers of this form: 4 = 2^3/2, 16 = 2^5/2, 32 = 4^3/2, 64 = 2^7/2, 108 = 6^3/2, 256 = 2^9/2 = 8^3/2, 500 = 10^3/2. - Jens Kruse Andersen, Jul 24 2014
The even terms of A070265, divided by two. - Jeppe Stig Nielsen, Jul 02 2017
LINKS
Jeppe Stig Nielsen, Generalized Fermat Primes sorted by base.
Eric Weisstein's World of Mathematics, Generalized Fermat Number.
MATHEMATICA
Module[{nn = 2^17, a = {}, n}, Do[If[b > nn, Break[], Do[If[Set[n, b^m/2] > nn, Break[], AppendTo[a, n]], {m, 3, Infinity, 2}]], {b, 2, Infinity, 2}]; Union@ a] (* Michael De Vlieger, Jul 04 2017 *)
PROG
(PARI) isOK(n)=ip=ispower(2*n); ip&&bitand(ip, ip-1) \\ Jeppe Stig Nielsen, Jul 02 2017
CROSSREFS
KEYWORD
easy,nonn
AUTHOR
Alexander Adamchuk, Feb 28 2007
EXTENSIONS
Definition changed by N. J. A. Sloane, Jul 26 2014 following the advice of Jens Kruse Andersen.
Terms after a(7) from Jeppe Stig Nielsen, Jul 02 2017
STATUS
approved