cp's OEIS Frontend

This is a front-end for the Online Encyclopedia of Integer Sequences, made by Christian Perfect. The idea is to provide OEIS entries in non-ancient HTML, and then to think about how they're presented visually. The source code is on GitHub.

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A242274 Numbers k such that k*3^k - 1 is semiprime.

Original entry on oeis.org

4, 5, 8, 12, 20, 24, 25, 28, 32, 38, 42, 44, 60, 62, 66, 70, 72, 80, 122, 125, 148, 228, 244, 270, 389, 390, 432, 464, 470, 488, 549, 560, 804, 862
Offset: 1

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Author

Vincenzo Librandi, May 12 2014

Keywords

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

Crossrefs

Cf. similar sequence listed in A242273.
Cf. A006553, A060352, A366323.

Programs

  • Magma
    IsSemiprime:=func; [n: n in [2..241] | IsSemiprime(s) where s is n*3^n-1];
  • Mathematica
    Select[Range[241], PrimeOmega[# 3^# - 1]==2&]
  • PARI
    isok(n)=bigomega(n*3^n-1)==2 /* Anders Hellström, Aug 18 2015 */

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
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