A068581 Let phi_m(x) = phi(phi(...(phi(x))...)) m times; sequence gives values of k such that phi_4(k) = tau(k).

1, 17, 23, 29, 31, 37, 43, 51, 55, 65, 69, 77, 82, 87, 91, 93, 94, 95, 106, 111, 118, 122, 128, 129, 133, 134, 136, 142, 146, 158, 165, 170, 184, 195, 218, 230, 231, 232, 238, 243, 246, 248, 250, 254, 273, 282, 285, 286, 290, 296, 297, 310, 318, 322, 344, 351

Offset: 1

Author

Benoit Cloitre, Mar 26 2002

Keywords

• nonn
• easy
• fini
• full

Comments

Last term is a(132) = 7560.

Numbers k such that A049100(k) = A000005(k).

Links

• Jinyuan Wang, Table of n, a(n) for n = 1..132

Crossrefs

Cf. A000005, A000010, A049100, A068579, A068580, A068582.

Programs

Mathematica

Select[Range[351], Nest[EulerPhi, #, 4] === DivisorSigma[0, #] &] (* Amiram Eldar, Jun 12 2022 *)