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A047821 Becomes prime after exactly 2 iterations of f(x) = sum of prime factors of x. 0
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 (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

Table of n, a(n) for n=1..59.

MATHEMATICA

i2Q[n_]:=PrimeQ[NestList[Total[Transpose[FactorInteger[#]][[1]]]&, n, 2]] == {False, False, True}; Select[Range[200], i2Q] (* Harvey P. Dale, Jul 14 2012 *)

CROSSREFS

Sequence in context: A038456 A346549 A075658 * A195238 A085816 A090066

Adjacent sequences: A047818 A047819 A047820 * A047822 A047823 A047824

KEYWORD

nonn

AUTHOR

David W. Wilson

STATUS

approved

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Last modified December 6 04:57 EST 2022. Contains 358594 sequences. (Running on oeis4.)