A077095 Numbers k such that iterating phi(sigma(k)-phi(k)) starting from k leads to the fixed point 24.

24, 42, 69, 74, 75, 82, 86, 94, 115, 125, 133, 155, 185, 187, 203, 289, 299, 323, 341, 361, 377, 437, 1681

Offset

1,1

Comments

Probably this sequence is finite, with 23 terms of which the last is 1681.

Links

Table of n, a(n) for n=1..23.

Example

n=1641: FixedPointList={1681,82,42,24}, end=24.

Mathematica

f[x_] := EulerPhi[DivisorSigma[1, x]-EulerPhi[x]]; Do[s=NestList[f, n, 100]; s1=Part[s, 99]; s2=Part[s, 100]; If[Equal[s1, s2]&&Equal[s1, 4], Print[{n, s1}]], {n, 1, 1000000}]
fp24Q[n_]:=FixedPoint[EulerPhi[DivisorSigma[1, #]-EulerPhi[#]]&, n, 20]==24; Select[ Range[1700], fp24Q] (* Harvey P. Dale, Mar 12 2023 *)

Crossrefs

Cf. A000010 (phi), A000203 (sigma).

Cf. A077090, A077091, A077092, A077093, A077094, A077096.

Sequence in context: A274350 A063702 A074975 * A228844 A111948 A039411

Adjacent sequences: A077092 A077093 A077094 * A077096 A077097 A077098

Keyword

nonn

Author

Labos Elemer, Oct 31 2002

Status

approved