A242339 Numbers k such that k*8^k-1 is semiprime.

3, 6, 9, 13, 21, 24, 32, 45, 61, 62, 73, 132, 150, 174, 187, 217, 266, 331, 534

Offset

1,1

Comments

The semiprimes of this form are: 1535, 1572863, 1207959551, 7146825580543, 193690812773950291967, 113336795588871485128703, 2535301200456458802993406410751, 1960026433464605549549037738806984897986559, ...

Links

Table of n, a(n) for n=1..19.

Mathematica

Select[Range[80], PrimeOmega[# 8^# - 1]==2&]

Prog

(Magma) IsSemiprime:=func<i | &+[d[2]: d in Factorization(i)] eq 2>; [n: n in [2..80] | IsSemiprime(s) where s is n*8^n-1];
(PARI) isok(n)=bigomega(n*8^n-1)==2 /* Anders Hellström, Aug 18 2015 */

Crossrefs

Cf. similar sequences listed in A242273.

Cf. A064754, A242201.

Sequence in context: A153006 A181970 A316531 * A086838 A161669 A128261

Adjacent sequences: A242336 A242337 A242338 * A242340 A242341 A242342

Keyword

nonn,more

Author

Vincenzo Librandi, May 12 2014

Extensions

a(12)-a(19) from Luke March, Aug 14 2015

Status

approved