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 A242175 Numbers n such that n*2^n + 1 is a semiprime. 3
 2, 3, 4, 5, 8, 9, 11, 16, 21, 33, 35, 101, 105, 131, 158, 165, 191, 234, 251, 435, 453, 459, 561, 579, 604, 671, 744, 753, 933, 963, 1041, 1146, 1168, 1254 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 COMMENTS The semiprimes of this form are 9, 25, 65, 161, 2049, 4609, 22529, ... (A242116). a(35) >= 1528. Below 2000, 1794 and 1961 are in the sequence. Unknown factorization for 1528, 1576, 1908. - Hugo Pfoertner, Jul 29 2019 LINKS FactorDB, Status of 1528*2^1528+1. FORMULA A002064(a(n)) = A242116(n). - Amiram Eldar, Nov 27 2019 MATHEMATICA Select[Range[165], Plus@@Last/@FactorInteger[# * 2^# + 1]==2&] PROG (MAGMA) IsSemiprime:=func; [n: n in [2..230] | IsSemiprime(s) where s is n*2^n+1]; // Bruno Berselli, May 08 2014 CROSSREFS Cf. A001358, A002064, A005849, A242116, A242273. Sequence in context: A044051 A083132 A302602 * A325662 A275771 A118956 Adjacent sequences:  A242172 A242173 A242174 * A242176 A242177 A242178 KEYWORD nonn,more,hard AUTHOR Vincenzo Librandi, May 07 2014 EXTENSIONS a(17) from Bruno Berselli, May 08 2014 a(18)-a(30) from Luke March, Aug 13 2015 a(31)-a(34) from Hugo Pfoertner, Jul 29 2019 Wrong term 941 removed by Amiram Eldar, Nov 27 2019 STATUS approved

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Last modified June 6 18:59 EDT 2020. Contains 334832 sequences. (Running on oeis4.)