A068579 Let phi_m(x) = phi(phi(...(phi(x))...)) m times; sequence gives values of k such that phi_3(k) = tau(k).

1, 11, 13, 19, 33, 34, 35, 39, 46, 57, 58, 62, 74, 86, 88, 102, 104, 105, 110, 130, 135, 138, 152, 154, 174, 182, 186, 190, 222, 258, 264, 280, 312, 330, 342, 390, 456, 462, 546, 570, 594, 756, 840, 1080

Offset: 1

Author

Benoit Cloitre, Mar 26 2002

Keywords

• nonn
• easy
• fini
• full

Comments

Numbers k such that A049099(k) = A000005(k).

Crossrefs

Cf. A000005, A000010, A049099, A068580, A068581, A068582.

Programs

Mathematica

Select[Range[1080], Nest[EulerPhi, #, 3] === DivisorSigma[0, #] &] (* Amiram Eldar, Jun 12 2022 *)

PARI

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