The OEIS is supported by the many generous donors to the OEIS Foundation.
%I #14 Sep 08 2022 08:46:08
%S 1,8,12,16,20,50,208,254,282,342,350,386
%N Numbers n such that n*9^n+1 is semiprime.
%C The semiprimes of this form are: 10, 344373769, 3389154437773, 29648323021629457, 243153309181138576021, ...
%C a(13) >= 512. - _Hugo Pfoertner_, Aug 05 2019
%H factordb.com, <a href="http://factordb.com/index.php?query=9%5E512*512%2B1">Status of 512*9^512+1</a>.
%t Select[Range[70], PrimeOmega[# 9^# + 1] == 2 &]
%o (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];
%Y Cf. similar sequences listed in A242203.
%Y Cf. A064747.
%K nonn,more,hard
%O 1,2
%A _Vincenzo Librandi_, May 10 2014
%E a(7)-a(12) from _Hugo Pfoertner_, Aug 05 2019