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%I #49 Sep 08 2022 08:46:08
%S 5,7,8,9,10,12,18,20,25,32,37,39,72,80,85,90,97,142,150,159,163,168,
%T 169,186,192,211,231,272,305,349,363,369,375,463,465,615,668,672,789,
%U 797,817,859,908,938,951,1092,1123
%N Numbers n such that n*2^n - 1 is a semiprime.
%C The semiprimes of this form are: 159, 895, 2047, 4607, 10239, ... (A242115).
%C a(48) >= 1152. - _Hugo Pfoertner_, Jul 29 2019
%H FactorDB, <a href="http://factordb.com/index.php?query=1152*2%5E1152-1">Status of 1152*2^1152-1</a>.
%F A003261(a(n)) = A242115(n). - _Amiram Eldar_, Nov 27 2019
%t Select[Range[1000], PrimeOmega[# 2^# - 1]==2&]
%o (Magma) IsSemiprime:=func<i | &+[d[2]: d in Factorization(i)] eq 2>; [n: n in [2..1000] | IsSemiprime(s) where s is n*2^n-1];
%Y Cf. numbers n such that n*k^n - 1 is semiprime: this sequence (k=2), A242274 (k=3), A242335 (k=4), A242336 (k=5), A242337 (k=6), A242338 (k=7), A242339 (k=8), A242340 (k=9), A242341 (k=10).
%Y Cf. A002234, A003261, A242115, A242175.
%K nonn,more,hard
%O 1,1
%A _Vincenzo Librandi_, May 12 2014
%E a(28)-a(29) from _Luke March_, Aug 05 2015
%E a(30)-a(42) from _Carl Schildkraut_, Aug 18 2015
%E Corrected and extended by _Luke March_, Sep 01 2015
%E Missing terms a(26)-a(27) inserted by _Amiram Eldar_, Nov 27 2019
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