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A047821
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Becomes prime after exactly 2 iterations of f(x) = sum of prime factors of x.
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0
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14, 15, 21, 28, 30, 35, 39, 42, 45, 46, 51, 55, 56, 57, 60, 63, 65, 66, 75, 77, 78, 84, 85, 87, 90, 91, 92, 93, 94, 95, 98, 102, 110, 111, 112, 114, 117, 119, 120, 123, 126, 130, 132, 135, 141, 143, 145, 147, 150, 153, 154, 155, 156, 158, 168, 170, 171, 174, 175
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OFFSET
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1,1
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COMMENTS
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f(x) = sum of prime factors without multiplicity, so that f(1500) = 2+3+5 = 10.
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LINKS
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MATHEMATICA
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i2Q[n_]:=PrimeQ[NestList[Total[Transpose[FactorInteger[#]][[1]]]&, n, 2]] == {False, False, True}; Select[Range[200], i2Q] (* Harvey P. Dale, Jul 14 2012 *)
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CROSSREFS
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KEYWORD
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nonn
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AUTHOR
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STATUS
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approved
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