A242272 Numbers n such that n*9^n+1 is semiprime.

1, 8, 12, 16, 20, 50, 208, 254, 282, 342, 350, 386

Offset: 1

Author

Vincenzo Librandi, May 10 2014

Keywords

• nonn
• more
• hard

Comments

The semiprimes of this form are: 10, 344373769, 3389154437773, 29648323021629457, 243153309181138576021, ...

a(13) >= 512. - Hugo Pfoertner, Aug 05 2019

Links

• factordb.com, Status of 512*9^512+1.

Crossrefs

Cf. similar sequences listed in A242203.

Cf. A064747.

Programs

Magma

IsSemiprime:=func; [n: n in [1..70] | IsSemiprime(s) where s is n*9^n+1];

Mathematica

Select[Range[70], PrimeOmega[# 9^# + 1] == 2 &]

Extensions

a(7)-a(12) from Hugo Pfoertner, Aug 05 2019