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A242273
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Numbers n such that n*2^n - 1 is a semiprime.
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12
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5, 7, 8, 9, 10, 12, 18, 20, 25, 32, 37, 39, 72, 80, 85, 90, 97, 142, 150, 159, 163, 168, 169, 186, 192, 211, 231, 272, 305, 349, 363, 369, 375, 463, 465, 615, 668, 672, 789, 797, 817, 859, 908, 938, 951, 1092, 1123
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OFFSET
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1,1
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COMMENTS
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The semiprimes of this form are: 159, 895, 2047, 4607, 10239, ... (A242115).
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LINKS
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FORMULA
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MATHEMATICA
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Select[Range[1000], PrimeOmega[# 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..1000] | IsSemiprime(s) where s is n*2^n-1];
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CROSSREFS
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KEYWORD
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nonn,more,hard
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AUTHOR
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EXTENSIONS
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Missing terms a(26)-a(27) inserted by Amiram Eldar, Nov 27 2019
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STATUS
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approved
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