A242340 Numbers k such that k*9^k-1 is semiprime.

2, 3, 6, 8, 11, 18, 28, 38, 48, 168, 254, 274, 310, 430

Offset

1,1

Comments

The semiprimes of this form are: 161, 2186, 3188645, 344373767, 345191655698, 2701703435345984177, 14653733724766095041978322587, ...

a(15) >= 512. - Daniel Suteu, Aug 05 2019

Links

Table of n, a(n) for n=1..14.

Mathematica

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

Prog

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

Crossrefs

Cf. similar sequences listed in A242273.

Cf. A064755, A242202.

Sequence in context: A342493 A353903 A325547 * A033766 A129340 A276588

Adjacent sequences: A242337 A242338 A242339 * A242341 A242342 A242343

Keyword

nonn,more

Author

Vincenzo Librandi, May 12 2014

Extensions

a(10)-a(14) from Daniel Suteu, Aug 05 2019

Status

approved