%I #24 Jun 18 2022 04:04:43
%S 9,33,65,129,2049,4097,8193,131073,524289,1048577,8388609,268435457,
%T 2147483649,4294967297,1099511627777,8796093022209,
%U 2305843009213693953,18446744073709551617,604462909807314587353089,4951760157141521099596496897
%N Semiprimes of the form 2^k + 1.
%H Amiram Eldar, <a href="/A092562/b092562.txt">Table of n, a(n) for n = 1..40</a> (terms 1..30 from Vincenzo Librandi)
%F a(n) = A000051(A092559(n)). - _Amiram Eldar_, Jun 18 2022
%e 2049 is a member because 2049 = 2^11+1 = 3*683.
%t Select[Table[2^n + 1, {n, 100}], PrimeOmega[#] == 2&] (* _Vincenzo Librandi_, Sep 21 2012 *)
%o (Magma) IsSemiprime:=func<n | &+[d[2]: d in Factorization(n)] eq 2>; [s: n in [2..100] | IsSemiprime(s) where s is 2^n+1]; // _Vincenzo Librandi_, Sep 21 2012
%Y Corresponding k: A092559.
%Y Cf. A000051, A092558, A092561.
%K nonn
%O 1,1
%A _Zak Seidov_, Feb 27 2004