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

401408, 414050, 436032, 455625, 462400, 466608, 476100, 486300, 486900, 512337, 522242, 526974, 543600, 544644, 544944, 546192, 546861, 554304, 559504, 571536, 572313, 575028, 577200, 579856, 583200, 585528, 599694, 604300, 609429, 611618

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, A096887, A095953, A096526, A095954, A096888, A096889, 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[624900], f[ # ] == 18 &]

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