OFFSET
1,1
COMMENTS
The semiprimes of this form are 9, 25, 65, 161, 2049, 4609, 22529, ... (A242116).
a(35) >= 1528. Below 2000, 1794 and 1961 are in the sequence. Unknown factorization for 1528, 1576, 1908. - Hugo Pfoertner, Jul 29 2019
LINKS
FORMULA
MATHEMATICA
Select[Range[165], Plus@@Last/@FactorInteger[# * 2^# + 1]==2&]
PROG
(Magma) IsSemiprime:=func<i | &+[d[2]: d in Factorization(i)] eq 2>; [n: n in [2..230] | IsSemiprime(s) where s is n*2^n+1]; // Bruno Berselli, May 08 2014
CROSSREFS
KEYWORD
nonn,more,hard
AUTHOR
Vincenzo Librandi, May 07 2014
EXTENSIONS
a(17) from Bruno Berselli, May 08 2014
a(18)-a(30) from Luke March, Aug 13 2015
a(31)-a(34) from Hugo Pfoertner, Jul 29 2019
Wrong term 941 removed by Amiram Eldar, Nov 27 2019
STATUS
approved