OFFSET
1,2
COMMENTS
The semiprimes of this form are 4, 82, 590491, 688747537, 69735688021, 690383311399, 6778308875545, 567024177788663347, 158049650967740074414, 29307467449532190083956645177, ...
a(23) >= 894. - Hugo Pfoertner, Aug 03 2019
MATHEMATICA
Select[Range[130], PrimeOmega[# 3^# + 1] == 2 &]
PROG
(Magma) IsSemiprime:=func<i | &+[d[2]: d in Factorization(i)] eq 2>; [n: n in [1..130] | IsSemiprime(s) where s is n*3^n+1];
(PARI) isok(n) = bigomega(n*3^n + 1)==2; \\ Michel Marcus, Mar 30 2019
CROSSREFS
KEYWORD
nonn,more
AUTHOR
Vincenzo Librandi, May 10 2014
EXTENSIONS
a(14)-a(20) from Luke March, Jul 30 2015
a(21)-a(22) from Daniel Suteu, Mar 30 2019
STATUS
approved