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A096526 Initial values for f(x)=phi(sigma(x)) such that iteration of f ends in a cycle of length 4. 10

%I

%S 324,400,484,490,530,544,576,630,660,672,684,690,714,722,750,756,768,

%T 770,772,777,780,792,804,810,819,828,832,833,840,841,846,852,858,864,

%U 868,870,872,876,888,892,901,906,910,918,920,924,930,936,940,948,952

%N Initial values for f(x)=phi(sigma(x)) such that iteration of f ends in a cycle of length 4.

%H Charles R Greathouse IV, <a href="/A096526/b096526.txt">Table of n, a(n) for n = 1..10000</a>

%e n=324: trajectory=324, 660, [576, 1512, 1280, 864], 576, ...}

%t g[n_] := EulerPhi[ DivisorSigma[1, n]]; f[n_] := f[n] = Block[{lst = NestWhileList[g, n, UnsameQ, All ]}, -Subtract @@ Flatten[ Position[lst, lst[[ -1]]]]]; Select[ Range[1000], f[ # ] == 4 &] (* _Robert G. Wilson v_, Jul 14 2004 *)

%o (PARI) f(x)=eulerphi(sigma(x))

%o is(n)=my(t=f(n), h=f(t)); while(t!=h, t=f(t); h=f(f(h))); h=f(h); t!=h && t!=(h=f(h)) && t==f(f(h)) \\ _Charles R Greathouse IV_, Nov 22 2013

%Y Cf. A000010, A000203, A095952, A096887, A095953, A095954, A096888, A096889, A096890, A095955, A095956.

%K nonn

%O 1,1

%A _Labos Elemer_, Jul 13 2004

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Last modified January 28 09:52 EST 2020. Contains 331319 sequences. (Running on oeis4.)