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 A047820 Becomes prime after exactly 1 iteration of f(x) = sum of prime factors of x. 3
 4, 6, 8, 9, 10, 12, 16, 18, 20, 22, 24, 25, 27, 32, 34, 36, 40, 44, 48, 49, 50, 54, 58, 64, 68, 72, 80, 81, 82, 88, 96, 100, 108, 116, 118, 121, 125, 128, 136, 142, 144, 160, 162, 164, 165, 169, 176, 192, 200, 202, 210, 214, 216, 232, 236, 242, 243, 250, 256, 272 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 COMMENTS f(x) = sum of prime factors without multiplicity, so that f(1500) = 2+3+5 = 10. LINKS MATHEMATICA Select[ Range@280, (fi = FactorInteger@#; Plus @@ Last /@ fi > 1 && PrimeQ[Plus @@ First /@ fi]) &] (from Robert G. Wilson v, Dec 09 2005) CROSSREFS Sequence in context: A067013 A141607 A071070 * A034878 A173328 A116661 Adjacent sequences:  A047817 A047818 A047819 * A047821 A047822 A047823 KEYWORD nonn AUTHOR STATUS approved

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