A047828 Becomes prime after exactly 9 iterations of f(x) = sum of prime factors of x.

41037, 55694, 82083, 86662, 88982, 90177, 106221, 106562, 109838, 111388, 123111, 134098, 168049, 173324, 177697, 177964, 179995, 195458, 207574, 213124, 214779, 215994, 219676, 222776, 229315, 233857, 240070, 240115, 244534, 246249

Offset

1,1

Comments

f(x) = sum of prime factors without multiplicity, so that f(1500) = 2+3+5 = 10.

Links

Harvey P. Dale, Table of n, a(n) for n = 1..5000

Mathematica

spf9Q[n_]:=Module[{spf=TakeDrop[NestList[Total[FactorInteger[#][[All, 1]]]&, n, 9], 9]}, AllTrue[spf[[1]], !PrimeQ[#]&]&&PrimeQ[spf][[2, 1]]]; Select[ Range[ 250000], spf9Q] (* Requires Mathematica version 10 or later *) (* Harvey P. Dale, Apr 20 2017 *)

Crossrefs

Sequence in context: A133863 A033532 A173361 * A141711 A074379 A237395

Adjacent sequences: A047825 A047826 A047827 * A047829 A047830 A047831

Keyword

nonn

Author

David W. Wilson

Status

approved