A096889 - OEIS

%I #6 Nov 25 2013 12:50:32

%S 467856,480200,490000,499968,514416,523344,524352,531441,548400,

%T 549444,550512,553728,556752,560532,562500,562800,570000,570276,

%U 570576,573744,576240,579024,579900,581700,584112,584836,586756,590268,595008,599076

%N Initial values for f(x)=phi(sigma(x)) such that iteration of f ends in cycle of length=15.

%H Charles R Greathouse IV, <a href="/A096889/b096889.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[599800], f[ # ] == 15 &]

%o (PARI) f(x,k=1)=for(i=1,k,x=eulerphi(sigma(x))); 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,2)) && t!=(h=f(h,2)) && t==f(h,10) \\ _Charles R Greathouse IV_, Nov 25 2013

%Y Cf. A000010, A000203, A095952, A096887, A095953, A096526, A095954, A096888, A096890, A095955, A095956.

%K nonn

%O 1,1

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