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A083647
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For primes p: Number of steps to reach 2 when iterating f(p) = greatest prime divisor of p-1.
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3
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0, 1, 1, 2, 2, 2, 1, 2, 3, 3, 2, 2, 2, 3, 4, 3, 4, 2, 3, 3, 2, 3, 3, 3, 2, 2, 2, 4, 2, 3, 3, 3, 2, 4, 3, 2, 3, 2, 4, 4, 4, 2, 3, 2, 3, 3, 3, 3, 4, 3, 4, 2, 2, 2, 1, 4, 4, 2, 4, 3, 5, 3, 2, 3, 3, 4, 3, 3, 5, 4, 3, 5, 3, 3, 3, 4, 3, 3, 2, 2, 3, 3, 4, 2, 3, 2, 3, 3, 4, 3, 5, 3, 2, 3, 4, 3, 4, 3, 4, 2, 3, 5, 4, 4, 3
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OFFSET
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1,4
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COMMENTS
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For smallest prime that requires n steps to reach 2 cf. A082449.
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LINKS
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EXAMPLE
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59 is the 17th prime and takes four steps to reach 2 (59 -> 29 -> 7 ->3 -> 2), so a(17) = 4.
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MATHEMATICA
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Table[Length[NestWhileList[FactorInteger[#-1][[-1, 1]]&, Prime[n], #!=2&]]-1, {n, 110}] (* Harvey P. Dale, Feb 27 2012 *)
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PROG
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(PARI) {forprime(p=2, 571, q=p; c=0; while(q>2, fac=factor(q-1); q=fac[matsize(fac)[1], 1]; c++); print1(c, ", "))}
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CROSSREFS
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KEYWORD
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nonn
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AUTHOR
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STATUS
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approved
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