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A048126 Becomes prime or 4 after exactly 4 iterations of f(x) = sum of prime factors of x. 2
26, 33, 39, 44, 49, 51, 55, 65, 66, 70, 77, 78, 84, 91, 92, 95, 98, 100, 105, 110, 112, 114, 119, 120, 125, 126, 128, 129, 130, 132, 135, 140, 143, 144, 150, 154, 156, 160, 162, 168, 170, 180, 189, 192, 196, 200, 204, 205, 216, 217, 220, 225, 240, 242, 243 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

COMMENTS

f(x) = sum of prime factors with multiplicity, so that f(1500) = 2+2+3+5+5+5 = 22.

LINKS

Harvey P. Dale, Table of n, a(n) for n = 1..1000

MATHEMATICA

spf[n_]:=Total[Flatten[Table[#[[1]], #[[2]]]&/@FactorInteger[n]]]; p4Q[n_] := Module[{lst=NestList[spf, n, 4]}, NoneTrue[Most[lst], PrimeQ] && NoneTrue[ Most[lst], #==4&]&&(PrimeQ[lst[[-1]]]||lst[[-1]]==4)]; Select[ Range[ 250], p4Q] (* Requires Mathematica version 10 or later *) (* Harvey P. Dale, Oct 17 2020 *)

CROSSREFS

Sequence in context: A050770 A050438 A047822 * A080932 A129914 A106794

Adjacent sequences: A048123 A048124 A048125 * A048127 A048128 A048129

KEYWORD

nonn

AUTHOR

David W. Wilson

STATUS

approved

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Last modified February 7 19:43 EST 2023. Contains 360128 sequences. (Running on oeis4.)