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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.)