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Let phi_m(x) = phi(phi(...(phi(x))...)) m times; sequence gives values of k such that phi_2(k) = tau(k).
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%I #17 Jun 12 2022 02:58:44

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

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

%C Numbers k such that A010554(k) = A000005(k).

%t Select[Range[210], Nest[EulerPhi, #, 2] === DivisorSigma[0, #] &] (* _Amiram Eldar_, Jun 12 2022 *)

%o (PARI) is(k) = numdiv(k) == eulerphi(eulerphi(k)); \\ _Jinyuan Wang_, Apr 05 2020

%Y Cf. A000005, A000010, A010554, A068579, A068581, A068582.

%K nonn,easy,fini,full

%O 1,2

%A _Benoit Cloitre_, Mar 26 2002