A373435 Iterate the function x <- phi(sigma(x)). The sequence has the smallest member of cycles of length 2.

4, 48, 72, 432, 1728, 10368, 184320, 1658880, 6220800, 10222080, 12856320000

Offset

1,1

Comments

A cycle of length 2 also starts at 3852635996160.

Links

Table of n, a(n) for n=1..11.

Example

phi(sigma(4)) = 6 and phi(sigma(6)) = 4, so 4 (the smallest term) is in the sequence.

Mathematica

Select[Range[10^6], # == EulerPhi[DivisorSigma[1, EulerPhi[DivisorSigma[1, #]]]] && # < EulerPhi[DivisorSigma[1, #]]&] (* Stefano Spezia, Jun 07 2024 *)

Prog

(PARI) isok(x) = my(y = eulerphi(sigma(x))); if (y > x, x == eulerphi(sigma(y))); \\ Michel Marcus, Jun 06 2024

Crossrefs

Cf. A000010, A000203, A062401, A001229, A095955. A095956, A373453, A373454.

Subsequence of A067883.

Sequence in context: A242225 A157818 A362402 * A048608 A366492 A275033

Adjacent sequences: A373432 A373433 A373434 * A373436 A373437 A373438

Keyword

nonn,more

Author

Jud McCranie, Jun 06 2024

Status

approved