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A047823 Becomes prime after exactly 4 iterations of f(x) = sum of prime factors of x. 1
62, 69, 124, 133, 190, 194, 201, 206, 207, 213, 217, 219, 226, 235, 238, 248, 249, 254, 282, 286, 303, 309, 310, 314, 355, 380, 386, 388, 406, 411, 412, 426, 447, 451, 452, 458, 469, 476, 483, 489, 496, 505, 508, 553, 564, 572, 598, 602, 603, 606, 620, 621 (list; graph; refs; listen; history; text; internal format)
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..1000

MATHEMATICA

p4iQ[n_]:=PrimeQ[NestList[Total[Transpose[FactorInteger[#]][[1]]]&, n, 4]] == {False, False, False, False, True}; Select[Range[700], p4iQ] (* Harvey P. Dale, Aug 18 2013 *)

CROSSREFS

Sequence in context: A024804 A025340 A048127 * A294738 A044997 A138172

Adjacent sequences:  A047820 A047821 A047822 * A047824 A047825 A047826

KEYWORD

nonn

AUTHOR

David W. Wilson

STATUS

approved

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Last modified November 14 06:40 EST 2019. Contains 329110 sequences. (Running on oeis4.)