A068580 Let phi_m(x) = phi(phi(...(phi(x))...)) m times; sequence gives values of k such that phi_2(k) = tau(k).

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

Offset: 1

Author

Benoit Cloitre, Mar 26 2002

Keywords

• nonn
• easy
• fini
• full

Comments

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

Crossrefs

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

Programs

Mathematica

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

PARI

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