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A048124
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Becomes prime or 4 after exactly 2 iterations of f(x) = sum of prime factors of x.
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0
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8, 9, 21, 25, 30, 32, 35, 36, 42, 50, 57, 60, 64, 72, 81, 85, 86, 93, 102, 111, 115, 121, 122, 138, 145, 146, 159, 164, 174, 182, 187, 194, 215, 219, 235, 236, 237, 253, 258, 260, 265, 266, 282, 284, 287, 289, 302, 303, 308, 312, 318, 319, 326, 329, 338, 346
(list; graph; refs; listen; history; internal format)
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OFFSET
| 1,1
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COMMENTS
| f(x) = sum of prime factors with multiplicity, so that f(1500) = 2+2+3+5+5+5 = 22.
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MATHEMATICA
| okQ[n_]:=Module[{lst=NestList[Total[Times@@@FactorInteger[#]]&, n, 2]}, !PrimeQ[First[lst]] &&!PrimeQ[lst[[2]]]&&First[lst]!=4&&lst[[2]]!=4&&(PrimeQ[Last[lst]]||Last[lst]==4)]; Select[Range[400], okQ] (* From Harvey P. Dale, Mar 23 2011 *)
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CROSSREFS
| Sequence in context: A120311 A061414 A071869 * A046258 A074344 A081351
Adjacent sequences: A048121 A048122 A048123 * A048125 A048126 A048127
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KEYWORD
| nonn
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AUTHOR
| David W. Wilson (davidwwilson(AT)comcast.net)
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