A376256 - OEIS

A376256

Numbers which are the minimum of a cycle in the map x -> phi(sigma(x)).

1, 2, 4, 8, 12, 16, 48, 72, 128, 240, 432, 576, 720, 1200, 1728, 1800, 6912, 10368, 15552, 27648, 32768, 41472, 67392, 142560, 184320, 272160, 326592, 712800, 1140480, 1190400, 1658880, 3345408, 3571200, 5702400, 6220800, 10222080, 14859936, 29719872, 40255488, 50319360, 113218560, 118879488

(list; graph; refs; listen; history; text; internal format)

OFFSET

1,2

COMMENTS

No further terms < 254731536.

The status of 254731536 is unknown, but conjectured not a term.

Additional terms include 2142720000, 5033164800, 150493593600, 3852635996160.

See further cycles in the linked document here below which contains 271 cycles, in Appendix C. It includes the 80 cycles complied by Jud McCrainie in a linked document at A095955.

LINKS

Table of n, a(n) for n=1..42.

Richard R. Forberg, Collecting Meta-Data About phi(sigma(n)) Cycles and Finding Many More Cycles, Oct 14, 2024.

PROG

(PARI) \\ Naive program, assumes eventual termination (ok upto 254731535).

isok(n)={my(M=Map(), p=n); while(!mapisdefined(M, p) && p>=n, mapput(M, p, 1); p=eulerphi(sigma(p))); p==n} \\ Andrew Howroyd, Sep 19 2024

CROSSREFS

Union of A001229, A373435, A373453, A373454, etc.

Cf. A062401, A095955.

Sequence in context: A368507 A273109 A046843 * A373728 A292060 A152125

Adjacent sequences: A376253 A376254 A376255 * A376257 A376258 A376259

KEYWORD

nonn

AUTHOR

Richard R. Forberg, Sep 16 2024

STATUS

approved