OFFSET
1,1
COMMENTS
The semiprimes of this form are 323, 1214, 52487, 6377291, 69735688019, 6778308875543, 21182215236074, 640550188738907, 59296646043258911, ...
804 is a term of this sequence. - Luke March, Aug 22 2015
The smallest unresolved value of k is now 862. - Sean A. Irvine, Jun 20 2022
The smallest unresolved value of k is now 866. - Tyler Busby, Oct 06 2023
From Jon E. Schoenfield, Oct 06 2023: (Start)
After the possible term 866, the only remaining 3-digit terms are 912 and 984, unless 920 is a term.
If k is an odd term, then k*3^k - 1 is even, so (k*3^k - 1)/2 is a prime. The next odd terms after 549 are 1125 and 12889. Odd terms are in A366323. (End)
26925 is a term. - Michael S. Branicky, Oct 08 2024
MATHEMATICA
Select[Range[241], PrimeOmega[# 3^# - 1]==2&]
PROG
(Magma) IsSemiprime:=func<i | &+[d[2]: d in Factorization(i)] eq 2>; [n: n in [2..241] | IsSemiprime(s) where s is n*3^n-1];
(PARI) isok(n)=bigomega(n*3^n-1)==2 /* Anders Hellström, Aug 18 2015 */
CROSSREFS
KEYWORD
nonn,more
AUTHOR
Vincenzo Librandi, May 12 2014
EXTENSIONS
a(21)-a(23) from Carl Schildkraut, Aug 18 2015
a(24)-a(32) from Luke March, Aug 22 2015
a(32) = 804 removed by Sean A. Irvine, Apr 25 2022
a(32)-a(33) from Sean A. Irvine, Jun 20 2022
a(34) from Tyler Busby, Oct 06 2023
STATUS
approved