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

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

Offset

1,2

Comments

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

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

Links

Table of n, a(n) for n=1..12.

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

Mathematica

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

Prog

(Magma) IsSemiprime:=func<i | &+[d[2]: d in Factorization(i)] eq 2>; [n: n in [1..70] | IsSemiprime(s) where s is n*9^n+1];

Crossrefs

Cf. similar sequences listed in A242203.

Cf. A064747.

Sequence in context: A192544 A302139 A160392 * A078609 A084809 A186407

Adjacent sequences: A242269 A242270 A242271 * A242273 A242274 A242275

Keyword

nonn,more,hard

Author

Vincenzo Librandi, May 10 2014

Extensions

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

Status

approved