A090896 a(n) = sigma^n(n), where sigma^n denotes functional iteration.

1, 4, 8, 24, 120, 1170, 1512, 15748, 15748, 61440, 196584, 1572840, 1098240, 15605760, 50328576, 7074432, 355532800, 2143289344, 13815410400, 224319234048, 33541980160, 109760857440, 1468475386560, 33151875434496, 1415960985600, 1510712608204800

Offset

1,2

Comments

a(8) = a(9). Are there any other numbers m, n such that a(m) = a(n)? a(16) < a(15); a(25) < a(24). How often is a(n+1) < a(n)?

a(88) = a(89), a(108) = a(109) and a(124) = a(125). - Seiichi Manyama, May 08 2021

Links

Seiichi Manyama, Table of n, a(n) for n = 1..200

Example

a(3) = sigma^3(3) = sigma^2(4) = sigma(7) = 8

Maple

a:= n-> (numtheory[sigma]@@n)(n):
seq(a(n), n=1..30);  # Alois P. Heinz, Oct 03 2018

Mathematica

a[n_] := Nest[DivisorSigma[1, #]&, n, n];
Table[a[n], {n, 1, 30}] (* Jean-François Alcover, Jan 30 2025 *)

Prog

(PARI) a(n) = my(k=sigma(n)); for(n=2, n, k=sigma(k)); k; \\ Seiichi Manyama, May 08 2021

Crossrefs

Main diagonal of A129246.

Cf. A000203, A101303.

Sequence in context: A303989 A052578 A066631 * A131637 A163143 A154586

Adjacent sequences: A090893 A090894 A090895 * A090897 A090898 A090899

Keyword

nonn,changed

Author

Gabriel Cunningham (gcasey(AT)mit.edu), Feb 25 2004

Status

approved