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

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

Offset

1,2

Comments

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

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

Links

Table of n, a(n) for n=1..10.

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

Mathematica

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

Prog

(Magma) IsSemiprime:=func<i | &+[d[2]: d in Factorization(i)] eq 2>; [n: n in [1..90] | IsSemiprime(s) where s is n*5^n+1];

Crossrefs

Cf. similar sequences listed in A242203.

Cf. A050916.

Sequence in context: A102545 A045521 A338766 * A076434 A192150 A049441

Adjacent sequences: A242202 A242203 A242204 * A242206 A242207 A242208

Keyword

nonn,more,hard

Author

Vincenzo Librandi, May 10 2014

Extensions

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

Status

approved