The OEIS is supported by the many generous donors to the OEIS Foundation.
%I #27 Mar 22 2023 08:13:59
%S 4,5,6,7,9,10,11,12,13,14,15,18,22,23,25,32,35,38,45,51,54,57,68,72,
%T 82,97,110,138,155,234,254
%N Numbers k such that k*6^k - 1 is semiprime.
%C The semiprimes of this form are 5183, 38879, 279935, 1959551, 90699263, 604661759, 3990767615, 26121388031, 169789022207, 1097098297343, ...
%C a(32) >= 423. - _Tyler Busby_, Mar 19 2023
%H factordb.com, <a href="http://factordb.com/index.php?query=6%5E423*423-1">Status of 423*6^423-1</a>.
%t Select[Range[352], PrimeOmega[# 6^# - 1]==2&]
%o (Magma) IsSemiprime:=func<i | &+[d[2]: d in Factorization(i)] eq 2>; [n: n in [2..352] | IsSemiprime(s) where s is n*6^n-1];
%Y Cf. similar sequences listed in A242273.
%Y Cf. A059675, A064752.
%K nonn,more
%O 1,1
%A _Vincenzo Librandi_, May 12 2014
%E a(26)-a(31) from _Carl Schildkraut_, Aug 17 2015