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A209292
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Non-semiprimes n such that 2n+1 are non-semiprimes.
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0
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1, 2, 3, 5, 8, 11, 13, 18, 20, 23, 29, 30, 31, 36, 37, 40, 41, 44, 48, 50, 52, 53, 54, 56, 63, 67, 68, 73, 75, 76, 78, 81, 83, 89, 90, 96, 97, 98, 99, 103, 105, 112, 113, 114, 116, 120, 125, 127, 128, 130, 131, 135, 136, 137, 138, 139, 140, 148, 153, 156
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OFFSET
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1,2
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COMMENTS
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LINKS
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FORMULA
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EXAMPLE
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a(1) = 1 because 1 is not a semiprime (the smallest semiprime is 4), and 2*1 + 1 = 3 is not a semiprime.
7 is not a semiprime, but 2*7 + 1 = 15 = 3*5 is a semiprime, so 7 is not in this sequence.
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MATHEMATICA
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SemiPrimeQ[n_Integer] := If[Abs[n] < 2, False, (2 == Plus @@ Transpose[FactorInteger[Abs[n]]][[2]])]; Select[Range[200], ! SemiPrimeQ[#] && ! SemiPrimeQ[2 # + 1] &] (* T. D. Noe, Jan 17 2013 *)
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PROG
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CROSSREFS
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KEYWORD
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nonn,easy
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AUTHOR
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STATUS
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approved
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