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A268110
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Numbers k such that (2^k-k+1)*2^k+1 is a semiprime.
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1
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3, 4, 6, 9, 10, 15, 19, 22, 26, 34, 47, 55, 67, 69, 72, 92, 100, 117, 160
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OFFSET
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1,1
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LINKS
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Table of n, a(n) for n=1..19.
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EXAMPLE
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a(1) = 3 because 6*8 +1 = 49 = 7*7, which is semiprime.
a(2) = 4 because 13*16+1 = 209 = 11*19, which is semiprime.
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MAPLE
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A268110:=n->`if`(numtheory[bigomega]((2^n-n+1)*2^n+1)=2, n, NULL): seq(A268110(n), n=1..80); # Wesley Ivan Hurt, Jan 30 2016
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MATHEMATICA
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Select[Range[105], PrimeOmega[(2^# - # + 1) 2^# + 1] == 2 &]
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PROG
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(Magma) IsSemiprime:=func<i | &+[d[2]: d in Factorization(i)] eq 2>; [n: n in [2..105]| IsSemiprime(s) where s is (2^n-n+1)*2^n+1];
(PARI) lista(nn) = {for(n=1, nn, if(bigomega((2^n-n+1)*2^n+1) == 2, print1(n, ", "))); } \\ Altug Alkan, Feb 07 2016
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CROSSREFS
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Cf. A201360: n for which (2^n-n+1)*2^n+1 is prime.
Sequence in context: A245810 A054686 A005214 * A124093 A025061 A284741
Adjacent sequences: A268107 A268108 A268109 * A268111 A268112 A268113
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KEYWORD
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nonn,more
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AUTHOR
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Vincenzo Librandi, Jan 30 2016
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EXTENSIONS
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a(18)-a(19) from Daniel Suteu, Aug 05 2019
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STATUS
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approved
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