This is a front-end for the Online Encyclopedia of Integer Sequences, made by Christian Perfect. The idea is to provide OEIS entries in non-ancient HTML, and then to think about how they're presented visually. The source code is on GitHub.
%I A068580 #17 Jun 12 2022 02:58:44 %S A068580 1,5,7,15,21,22,26,40,56,66,70,78,108,120,126,168,210 %N A068580 Let phi_m(x) = phi(phi(...(phi(x))...)) m times; sequence gives values of k such that phi_2(k) = tau(k). %C A068580 Numbers k such that A010554(k) = A000005(k). %t A068580 Select[Range[210], Nest[EulerPhi, #, 2] === DivisorSigma[0, #] &] (* _Amiram Eldar_, Jun 12 2022 *) %o A068580 (PARI) is(k) = numdiv(k) == eulerphi(eulerphi(k)); \\ _Jinyuan Wang_, Apr 05 2020 %Y A068580 Cf. A000005, A000010, A010554, A068579, A068581, A068582. %K A068580 nonn,easy,fini,full %O A068580 1,2 %A A068580 _Benoit Cloitre_, Mar 26 2002