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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A242203 Numbers n such that n*3^n + 1 is semiprime.

Original entry on oeis.org

1, 3, 10, 16, 20, 22, 24, 34, 39, 56, 63, 108, 128, 194, 202, 212, 214, 218, 314, 364, 662, 722
Offset: 1

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Author

Vincenzo Librandi, May 10 2014

Keywords

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

Links

Crossrefs

Cf. numbers n such that n*k^n + 1 is semiprime: A242175 (k=2), this sequence (k=3), A242204 (k=4), A242205 (k=5), A242269 (k=6), A242270 (k=7), A242271 (k=8), A242272 (k=9), A216378 (k=10).
Cf. A006552, A050914.

Programs

  • Magma
    IsSemiprime:=func; [n: n in [1..130] | IsSemiprime(s) where s is n*3^n+1];
  • Mathematica
    Select[Range[130], PrimeOmega[# 3^# + 1] == 2 &]
  • PARI
    isok(n) = bigomega(n*3^n + 1)==2; \\ Michel Marcus, Mar 30 2019

Extensions

a(14)-a(20) from Luke March, Jul 30 2015
a(21)-a(22) from Daniel Suteu, Mar 30 2019
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