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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
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
KEYWORD
nonn
AUTHOR
STATUS
approved