A242205 Numbers n such that n*5^n+1 is semiprime.

1, 2, 4, 16, 21, 40, 76, 113, 153, 288

Offset: 1

Author

Vincenzo Librandi, May 10 2014

Keywords

• nonn
• more
• hard

Comments

The semiprimes of this form are 6, 51, 2501, 2441406250001, 10013580322265626, 363797880709171295166015625001, ...

a(11) >= 1006. - Hugo Pfoertner, Aug 05 2019

Links

• factordb.com, Status of 1006*5^1006+1.

Crossrefs

Cf. similar sequences listed in A242203.

Cf. A050916.

Programs

Magma

IsSemiprime:=func; [n: n in [1..90] | IsSemiprime(s) where s is n*5^n+1];

Mathematica

Select[Range[90], PrimeOmega[# 5^# + 1] == 2 &]

Extensions

a(8)-a(10) from Luke March, Aug 13 2015