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A096890 Initial values for f(x)=phi(sigma(x)) such that iteration of f ends in a cycle of length 18. 13

%I #6 Nov 25 2013 12:59:03

%S 401408,414050,436032,455625,462400,466608,476100,486300,486900,

%T 512337,522242,526974,543600,544644,544944,546192,546861,554304,

%U 559504,571536,572313,575028,577200,579856,583200,585528,599694,604300,609429,611618

%N Initial values for f(x)=phi(sigma(x)) such that iteration of f ends in a cycle of length 18.

%H Charles R Greathouse IV, <a href="/A096890/b096890.txt">Table of n, a(n) for n = 1..10000</a>

%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[624900], f[ # ] == 18 &]

%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))); for(i=1,17,h=f(h); if(t==h,return(0))); t==f(h) \\ _Charles R Greathouse IV_, Nov 25 2013

%Y Cf. A000010, A000203, A095952, A096887, A095953, A096526, A095954, A096888, A096889, A095955, A095956.

%K nonn

%O 1,1

%A _Labos Elemer_ and _Robert G. Wilson v_, Jul 14 2004

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Last modified March 29 04:23 EDT 2024. Contains 371264 sequences. (Running on oeis4.)