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 A120685 Integers m such that the sequence defined by f(0)=m and f(n+1)=1+gpf(f(n)), with gpf(n) being the greatest prime factor of n (A006530), ends up in the repetitive cycle 4 -> 3 -> 3 -> ... 5

%I

%S 2,4,5,8,10,11,13,15,16,17,20,22,23,25,26,30,32,33,34,37,39,40,41,44,

%T 45,46,47,50,51,52,53,55,60,61,64,65,66,68,69,71,74,75,77,78,80,82,83,

%U 85,88,90,91,92,94,97,99,100,102,104,106,107,110,111,113,115,117,119,120

%N Integers m such that the sequence defined by f(0)=m and f(n+1)=1+gpf(f(n)), with gpf(n) being the greatest prime factor of n (A006530), ends up in the repetitive cycle 4 -> 3 -> 3 -> ...

%C Let f(0)=m; f(n+1)=1+gpf(f(n)), where gpf(n) is the greatest prime factor of n (A006530). For any m, for sufficiently large n the sequence f(n) oscillates between 3 and 4. Given a sufficiently large n, this allows us to divide integers in two classes: C3 (m such that the sequence f(n) enters the cycle 3, 4, 3, ...) and C4 (m such that the sequence f(n) enters the cycle 4, 3, 4, ...). We present here C4 as the one that begin with 4. In A120684 we present C3 as the one that begin with 3.

%e Oscillation between 3 and 4: 1+gpf(3)=1+3=4; 1+gpf(4)=1+2=3.

%e Other value, e.g. 7: 1+gpf(7)=1+7=8; 1+gpf(8)=1+2=3 (7 belongs to C3).

%e Other value, e.g. 20: 1+gpf(20)=1+5=6; 1+gpf(6)=1+3=4 (20 belongs to C4).

%t f = Function[n, FactorInteger[n][[ -1, 1]] + 1]; mn = Map[(NestList[f, #, 8][[ -1]]) &, Range[2, 500]]; out = Flatten[Position[mn, 4]] + 1

%Y Cf. A120684, A072268, A006530.

%K nonn

%O 0,1

%A _Carlos Alves_, Jun 25 2006

%E Edited by _Michel Marcus_, Feb 25 2013

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Last modified December 4 02:33 EST 2021. Contains 349468 sequences. (Running on oeis4.)