OFFSET
1,1
COMMENTS
a(48) >= 428. - Serge Batalov, Feb 25 2023
LINKS
factordb.com, Status of (2^428+1)^2-2.
EXAMPLE
a(1) = 4 because 17^2 - 2 = 287 = 7*41, which is semiprime.
a(2) = 6 because 65^2 - 2 = 4223 = 41*103, which is semiprime.
MATHEMATICA
Select[Range[105], PrimeOmega[(2^# + 1)^2 - 2] == 2 &]
PROG
(Magma) IsSemiprime:=func<i | &+[d[2]: d in Factorization(i)] eq 2>; [n: n in [1..110]| IsSemiprime(s) where s is (2^n+1)^2-2];
(PARI) isok(n) = bigomega((2^n+1)^2-2) == 2; \\ Michel Marcus, Feb 22 2016
CROSSREFS
KEYWORD
nonn,more,hard
AUTHOR
Vincenzo Librandi, Feb 21 2016
EXTENSIONS
a(25)-a(39) from Hugo Pfoertner, Aug 05 2019
a(40)-a(41) from chris2be8@yahoo.com, Feb 25 2023
a(42)-a(47) from Serge Batalov, Feb 26 2023
STATUS
approved