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 A048125 Becomes prime or 4 after exactly 3 iterations of f(x) = sum of prime factors of x. 2
 14, 15, 16, 18, 20, 24, 27, 38, 46, 68, 87, 124, 141, 152, 155, 158, 161, 166, 171, 183, 185, 186, 195, 203, 208, 209, 212, 218, 221, 222, 226, 230, 231, 232, 234, 247, 249, 255, 261, 272, 275, 276, 290, 295, 299, 301, 306, 314, 322, 323, 330, 335, 341, 343 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 COMMENTS f(x) = sum of prime factors with multiplicity, so that f(1500) = 2+2+3+5+5+5 = 22. LINKS Andrew Howroyd, Table of n, a(n) for n = 1..10000 MATHEMATICA f[n_] := Total[Times @@@ FactorInteger[n]]; okQ[n_] := For[k = 3; r = n, k >= 1, k--, r = f[r]; If[r == 4 || PrimeQ[r], Return[k == 1]]]; Select[Range[500], okQ] (* Jean-François Alcover, Sep 20 2019, from PARI *) PROG (PARI) sopfr(n)={my(f=factor(n)); sum(i=1, #f~, f[i, 1]*f[i, 2])} ok(n)={forstep(k=3, 1, -1, n=sopfr(n); if(n==4||isprime(n), return(k==1))); 0} select(ok, [1..500]) \\ Andrew Howroyd, Sep 14 2019 CROSSREFS Cf. A002217, A048124. Sequence in context: A296748 A270042 A332924 * A163482 A257782 A206447 Adjacent sequences: A048122 A048123 A048124 * A048126 A048127 A048128 KEYWORD nonn AUTHOR David W. Wilson STATUS approved

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Last modified September 28 19:19 EDT 2023. Contains 365737 sequences. (Running on oeis4.)