A096887 Initial values for f(x)=phi(sigma(x)) such that iteration of f ends in cycle of length=2.

4, 6, 7, 10, 11, 13, 17, 32, 36, 42, 48, 49, 50, 52, 54, 56, 57, 60, 62, 63, 64, 66, 67, 68, 69, 70, 72, 73, 74, 75, 76, 77, 78, 79, 80, 81, 82, 84, 85, 87, 88, 90, 91, 92, 93, 94, 95, 96, 97, 98, 99, 100, 101, 102, 103, 104, 105, 106, 107, 108, 110, 111, 112, 113, 114

Offset: 1

Author

Labos Elemer and Robert G. Wilson v, Jul 14 2004

Keywords

• nonn

Links

• Charles R Greathouse IV, Table of n, a(n) for n = 1..10000

Crossrefs

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

Programs

Mathematica

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 &]

PARI

f(x)=eulerphi(sigma(x))
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