A048128 Becomes prime or 4 after exactly 6 iterations of f(x) = sum of prime factors of x.

134, 177, 213, 262, 309, 334, 339, 398, 445, 469, 505, 514, 534, 606, 670, 723, 742, 753, 758, 763, 788, 793, 802, 804, 817, 818, 849, 865, 913, 914, 915, 926, 961, 976, 998, 1011, 1016, 1038, 1052, 1060, 1090, 1098, 1101, 1115, 1143, 1154, 1165, 1195

Offset: 1

Author

David W. Wilson

Keywords

• nonn

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..3500

Programs

Mathematica

nxt[n_]:=Total[Flatten[Table[#[[1]],{#[[2]]}]&/@FactorInteger[n]]]; sixiQ[ n_]:=Module[{c=NestList[nxt,n,6]},PrimeQ[c]=={False,False,False,False,False,False,True}  ||  FirstPosition[ c,4,1] == {7}]; Select[ Range[ 1200],sixiQ] (* Harvey P. Dale, Oct 21 2014 *)