A373453 Iterate the function x <- phi(sigma(x)). The sequence has the smallest member of cycles of length 3.

16, 1200, 15552, 67392, 272160, 69672960000

Offset

1,1

Comments

69672960000 is also a term in the sequence.

a(7) <= 2704853606400. The numbers 242595672883200000, 66217181184000000000 and 185577469193591193600 are also terms. - Giorgos Kalogeropoulos, Jun 18 2024

4672651788288000 is also a term. - Jud McCranie, Jun 18 2024

The sequence is complete through 10^13. - Jud McCranie, Sep 14 2024

Links

Table of n, a(n) for n=1..6.

Example

16 -> 30 -> 24 -> 16, so 16 (the smallest term) is in the sequence.

Prog

(PARI) isok(x) = my(y = eulerphi(sigma(x))); if (y > x, my(z = eulerphi(sigma(y))); if (z > x, x == eulerphi(sigma(z)))); \\ Michel Marcus, Jun 07 2024

Crossrefs

Cf. A001229, A095955, A095956, A373435, A373454.

Sequence in context: A053903 A254333 A102807 * A160131 A206691 A064346

Adjacent sequences: A373450 A373451 A373452 * A373454 A373455 A373456

Keyword

nonn,more,changed

Author

Jud McCranie, Jun 06 2024

Extensions

a(6) from Giorgos Kalogeropoulos, Jun 18 2024

Status

approved