A000458 a(0) = a(1) = 1; thereafter a(n) = sigma(a(n-1)) + sigma(a(n-2)).

1, 1, 2, 4, 10, 25, 49, 88, 237, 500, 1412, 3570, 12846, 36072, 126504, 493920, 2358720, 12292224, 49984224, 171237888, 642804078, 1980490350, 6380883000, 27032104440, 117961599600, 555861355920, 2623445776224

Offset

0,3

Links

T. D. Noe, Table of n, a(n) for n=0..100

Maple

A000458 := proc(n) options remember; if n <= 1 then RETURN(1) else sigma(A000458(n-1))+sigma(A000458(n-2)); fi; end;

Mathematica

a[0] = a[1] = 1; a[n_] := a[n] = DivisorSigma[1, a[n-1]] + DivisorSigma[1, a[n-2]]; Table[a[n], {n, 0, 30}] (* Jean-François Alcover, Feb 08 2016 *)
nxt[{a_, b_}]:={b, DivisorSigma[1, a]+DivisorSigma[1, b]}; NestList[nxt, {1, 1}, 30][[All, 1]] (* Harvey P. Dale, Dec 09 2022 *)

Crossrefs

Sequence in context: A148090 A148091 A123420 * A089928 A173610 A036887

Adjacent sequences: A000455 A000456 A000457 * A000459 A000460 A000461

Keyword

nonn

Author

N. J. A. Sloane

Status

approved