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A230305
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Iterate A080670 starting at n; a(n) = number of steps to reach a prime, or -1 if no prime is ever reached.
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6
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0, 0, 2, 0, 1, 0, 1, 4, 3, 0, 1, 0, 3, 4, 2, 0, 3, 0
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listen;
history;
text;
internal format)
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OFFSET
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2,3
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COMMENTS
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If n is a prime, a(n) = 0.
a(20) is presently unknown - see A195265 for the trajectory.
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LINKS
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EXAMPLE
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9 -> 32 -> 25 -> 52 -> 2213, which is prime, taking 4 steps, so a(9) = 4.
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MATHEMATICA
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fn[n_] := FromDigits[Flatten[IntegerDigits[DeleteCases[Flatten[FactorInteger[n]], 1]]]];
Map[Length, Table[NestWhileList[fn, n, # != 1 && ! PrimeQ[#] &], {n, 2,
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CROSSREFS
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KEYWORD
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nonn,base,more,hard
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AUTHOR
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STATUS
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approved
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