A096887 - OEIS

%I #6 Nov 22 2013 16:35:04

%S 4,6,7,10,11,13,17,32,36,42,48,49,50,52,54,56,57,60,62,63,64,66,67,68,

%T 69,70,72,73,74,75,76,77,78,79,80,81,82,84,85,87,88,90,91,92,93,94,95,

%U 96,97,98,99,100,101,102,103,104,105,106,107,108,110,111,112,113,114

%N Initial values for f(x)=phi(sigma(x)) such that iteration of f ends in cycle of length=2.

%H Charles R Greathouse IV, <a href="/A096887/b096887.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[115], f[ # ] == 2 &]

%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==f(h) \\ _Charles R Greathouse IV_, Nov 22 2013

%Y Cf. A000010, A000203, A095952, A095953, A096526, A095954, A096888, A096889, A096890, A095955, A095956.

%K nonn

%O 1,1

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