A242337 Numbers k such that k*6^k - 1 is semiprime.

4, 5, 6, 7, 9, 10, 11, 12, 13, 14, 15, 18, 22, 23, 25, 32, 35, 38, 45, 51, 54, 57, 68, 72, 82, 97, 110, 138, 155, 234, 254

Offset

1,1

Comments

The semiprimes of this form are 5183, 38879, 279935, 1959551, 90699263, 604661759, 3990767615, 26121388031, 169789022207, 1097098297343, ...

a(32) >= 423. - Tyler Busby, Mar 19 2023

Links

Table of n, a(n) for n=1..31.

factordb.com, Status of 423*6^423-1.

Mathematica

Select[Range[352], PrimeOmega[# 6^# - 1]==2&]

Prog

(Magma) IsSemiprime:=func<i | &+[d[2]: d in Factorization(i)] eq 2>; [n: n in [2..352] | IsSemiprime(s) where s is n*6^n-1];

Crossrefs

Cf. similar sequences listed in A242273.

Cf. A059675, A064752.

Sequence in context: A196697 A068318 A347932 * A201739 A329782 A218044

Adjacent sequences: A242334 A242335 A242336 * A242338 A242339 A242340

Keyword

nonn,more

Author

Vincenzo Librandi, May 12 2014

Extensions

a(26)-a(31) from Carl Schildkraut, Aug 17 2015

Status

approved