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A096526 Initial values for f(x)=phi(sigma(x)) such that iteration of f ends in a cycle of length 4. 10
324, 400, 484, 490, 530, 544, 576, 630, 660, 672, 684, 690, 714, 722, 750, 756, 768, 770, 772, 777, 780, 792, 804, 810, 819, 828, 832, 833, 840, 841, 846, 852, 858, 864, 868, 870, 872, 876, 888, 892, 901, 906, 910, 918, 920, 924, 930, 936, 940, 948, 952 (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
EXAMPLE
n=324: trajectory=324, 660, [576, 1512, 1280, 864], 576, ...}
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[1000], f[ # ] == 4 &] (* Robert G. Wilson v, Jul 14 2004 *)
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))); h=f(h); t!=h && t!=(h=f(h)) && t==f(f(h)) \\ Charles R Greathouse IV, Nov 22 2013
CROSSREFS
Sequence in context: A088214 A117550 A045287 * A111278 A241502 A014792
KEYWORD
nonn
AUTHOR
Labos Elemer, Jul 13 2004
STATUS
approved

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Last modified May 29 12:08 EDT 2024. Contains 372940 sequences. (Running on oeis4.)