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A230306
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Evil numbers (A001969) which can be written as a product of two evil numbers > 1.
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2
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9, 15, 18, 27, 30, 36, 45, 51, 54, 60, 72, 75, 85, 90, 99, 102, 108, 120, 129, 135, 144, 150, 153, 159, 165, 170, 180, 189, 195, 198, 204, 207, 215, 216, 225, 231, 240, 243, 249, 255, 258, 267, 270, 288, 297, 300, 303, 306, 315, 318, 325, 330, 340, 360, 378
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OFFSET
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1,1
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LINKS
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EXAMPLE
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Evil number 75 = 3*5*5. Since 3 and 5 are evil numbers, than 75 is in the sequence.
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MATHEMATICA
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evilQ[n_] := EvenQ[DigitCount[n, 2][[1]]]; fQ[n_] := Module[{f, i}, If[PrimeQ[n], False, f = Select[Divisors[n], # > 1 && # <= Sqrt[n] &]; i = 1; While[i <= Length[f] && ! (evilQ[f[[i]]] && evilQ[n/f[[i]]]), i++]; i <= Length[f]]]; Select[Range[1000], evilQ[#] && fQ[#] &] (* T. D. Noe, Oct 16 2013 *)
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CROSSREFS
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KEYWORD
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nonn,base
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AUTHOR
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EXTENSIONS
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STATUS
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approved
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