A067883 Numbers k such that k = phi(sigma(phi(sigma(k)))).

1, 2, 4, 6, 8, 12, 48, 60, 72, 96, 128, 240, 432, 480, 720, 1728, 2016, 6912, 10368, 14080, 32768, 142560, 184320, 196560, 712800, 1140480, 1190400, 1658880, 1801800, 3345408, 3571200, 5702400, 6220800, 8363520, 10222080, 10368000, 14859936, 29719872, 50319360

Offset: 1

Author

Benoit Cloitre, Mar 02 2002

Keywords

• easy
• nonn

Comments

This sequence has both terms of cycles of length 2 when iterating k <-phi(sigma(k)) (see A373435) plus cycles of length 1 (see A001229). - Jud McCranie, Aug 09 2024

Crossrefs

Cf. A000010, A000203, A062401, A163372.

Supersequence of A001229. A373435 is a subsequence of this sequence.

Programs

Mathematica

Select[Range[10^6], EulerPhi[DivisorSigma[1, EulerPhi[DivisorSigma[1, #]]]] == # &] (* Amiram Eldar, Apr 24 2022 *)

Extensions

More terms from Reiner Martin, Jul 09 2002

a(35)-a(39) from Amiram Eldar, Apr 24 2022