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

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

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 A379235

Adjacent sequences: A047818 A047819 A047820 * A047822 A047823 A047824

Keyword

nonn

Author

David W. Wilson

Status

approved