

A068580


Let phi_m(x) = phi(phi(...(phi(x))...)) m times; sequence gives values of k such that phi_2(k) = tau(k).


3



1, 5, 7, 15, 21, 22, 26, 40, 56, 66, 70, 78, 108, 120, 126, 168, 210
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OFFSET

1,2


COMMENTS



LINKS



MATHEMATICA

Select[Range[210], Nest[EulerPhi, #, 2] === DivisorSigma[0, #] &] (* Amiram Eldar, Jun 12 2022 *)


PROG

(PARI) is(k) = numdiv(k) == eulerphi(eulerphi(k)); \\ Jinyuan Wang, Apr 05 2020


CROSSREFS



KEYWORD

nonn,easy,fini,full


AUTHOR



STATUS

approved



