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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
 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 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 LINKS Charles R Greathouse IV, Table of n, a(n) for n = 1..10000 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 &] PROG (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 CROSSREFS Cf. A000010, A000203, A095952, A096887, A095953, A096526, A095954, A096888, A096889, A095955, A095956. Sequence in context: A190837 A205094 A234419 * A234056 A157826 A215998 Adjacent sequences:  A096887 A096888 A096889 * A096891 A096892 A096893 KEYWORD nonn AUTHOR Labos Elemer and Robert G. Wilson v, Jul 14 2004 STATUS approved

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Last modified October 29 04:54 EDT 2020. Contains 338066 sequences. (Running on oeis4.)