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A242340
Numbers k such that k*9^k-1 is semiprime.
1
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
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.
Sequence in context: A342493 A353903 A325547 * A033766 A129340 A276588
KEYWORD
nonn,more
AUTHOR
Vincenzo Librandi, May 12 2014
EXTENSIONS
a(10)-a(14) from Daniel Suteu, Aug 05 2019
STATUS
approved