%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