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%I #17 Sep 08 2022 08:46:08
%S 2,6,8,9,13,15,25,36,37,63,66,72,73,85,205,333,430
%N Numbers n such that n*4^n+1 is semiprime.
%C The semiprimes of this form are: 33, 24577, 524289, 2359297, 872415233, 16106127361, 28147497671065601, 170005193383307227693057, 698910239464707491627009, ...
%C a(18) >= 547. - _Hugo Pfoertner_, Aug 05 2019
%H factordb.com, <a href="http://factordb.com/index.php?query=547*4%5E547%2B1">Status of 547*4^547+1</a>.
%t Select[Range[120], PrimeOmega[# 4^# + 1] == 2 &]
%o (Magma) IsSemiprime:=func<i | &+[d[2]: d in Factorization(i)] eq 2>; [n: n in [1..110] | IsSemiprime(s) where s is n*4^n+1];
%Y Cf. similar sequences listed in A242203.
%Y Cf. A007646, A050915.
%K nonn,more,hard
%O 1,1
%A _Vincenzo Librandi_, May 10 2014
%E a(15)-a(17) from _Luke March_, Aug 13 2015