A048129 Becomes prime or 4 after exactly 7 iterations of f(x) = sum of prime factors of x.

393, 422, 614, 674, 692, 886, 889, 934, 993, 1006, 1270, 1285, 1502, 1522, 1524, 1542, 1726, 1822, 2018, 2217, 2326, 2386, 2402, 2474, 2545, 2654, 2733, 2734, 2761, 2858, 2876, 3005, 3022, 3039, 3054, 3147, 3193, 3261, 3453, 3507, 3512, 3518, 3589

Offset

1,1

Comments

f(x) = sum of prime factors with multiplicity, so that f(1500) = 2+2+3+5+5+5 = 22.

Links

Table of n, a(n) for n=1..43.

Mathematica

spf[n_]:=Total[Flatten[Table[#[[1]], {#[[2]]}]&/@FactorInteger[n]]]; pf7Q[ n_]:= Module[ {lst=NestList[spf, n, 7]}, NoneTrue[Most[lst], PrimeQ] && !MemberQ[Most[lst], 4]&&(PrimeQ[Last[lst]]||Last[lst]==4)]; Select[Range[ 3600], pf7Q] (* Harvey P. Dale, Jul 11 2014 *)

Crossrefs

Sequence in context: A068288 A085019 A105233 * A045194 A105210 A256742

Adjacent sequences: A048126 A048127 A048128 * A048130 A048131 A048132

Keyword

nonn

Author

David W. Wilson

Status

approved