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
KEYWORD
nonn
AUTHOR
Labos Elemer and Robert G. Wilson v, Jul 14 2004
STATUS
approved