A048126 Becomes prime or 4 after exactly 4 iterations of f(x) = sum of prime factors of x.

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

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