OFFSET
1,1
COMMENTS
The semiprimes of this form are: 10485759, 201326591, 1236950581247, 6755399441055743, 486388759756013567, 13617340432139183023890366463, ...
MATHEMATICA
Select[Range[100], PrimeOmega[# 4^# - 1]==2&]
PROG
(Magma) IsSemiprime:=func<i | &+[d[2]: d in Factorization(i)] eq 2>; [n: n in [2..100] | IsSemiprime(s) where s is n*4^n-1];
(PARI) isok(n)=bigomega(n*4^n-1)==2 \\ Anders Hellström, Aug 18 2015
CROSSREFS
KEYWORD
nonn
AUTHOR
Vincenzo Librandi, May 12 2014
EXTENSIONS
a(12)-a(16) from Carl Schildkraut, Aug 18 2015
a(17)-a(27) from Charles R Greathouse IV, Aug 18 2015
STATUS
approved