A242335 Numbers k such that k*4^k-1 is semiprime.

10, 12, 18, 24, 27, 44, 47, 65, 71, 82, 84, 131, 134, 138, 143, 155, 164, 168, 197, 212, 227, 243, 248, 293, 302, 384, 401

Offset: 1

Author

Vincenzo Librandi, May 12 2014

Keywords

• nonn

Comments

The semiprimes of this form are: 10485759, 201326591, 1236950581247, 6755399441055743, 486388759756013567, 13617340432139183023890366463, ...

Crossrefs

Cf. similar sequences listed in A242273.

Cf. A060416, A086661.

Programs

Magma

IsSemiprime:=func; [n: n in [2..100] | IsSemiprime(s) where s is n*4^n-1];

Mathematica

Select[Range[100], PrimeOmega[# 4^# - 1]==2&]

PARI

isok(n)=bigomega(n*4^n-1)==2 \\ Anders Hellström, Aug 18 2015

Extensions

a(12)-a(16) from Carl Schildkraut, Aug 18 2015

a(17)-a(27) from Charles R Greathouse IV, Aug 18 2015