A023891 Sum of composite divisors of n.

0, 0, 0, 4, 0, 6, 0, 12, 9, 10, 0, 22, 0, 14, 15, 28, 0, 33, 0, 34, 21, 22, 0, 54, 25, 26, 36, 46, 0, 61, 0, 60, 33, 34, 35, 85, 0, 38, 39, 82, 0, 83, 0, 70, 69, 46, 0, 118, 49, 85, 51, 82, 0, 114, 55, 110, 57, 58, 0, 157, 0, 62, 93, 124, 65, 127, 0, 106, 69, 129, 0

Offset

1,4

Links

Vincenzo Librandi, Table of n, a(n) for n = 1..1000

Formula

a(n) = A023890(n) - 1. - Sean A. Irvine, Jun 11 2019

a(n) = A000203(n) - A074372(n). - Omar E. Pol, Oct 28 2025

Mathematica

Array[ Plus @@ (Select[ Divisors[ # ], (!PrimeQ[ # ] && #>1)& ])&, 75 ]
a[n_] := DivisorSigma[1, n] - Plus @@ FactorInteger[n][[;; , 1]] - 1; a[1] = 0; Array[a, 100] (* Amiram Eldar, Jun 20 2022 *)

Prog

(PARI) a(n) = sumdiv(n, d, d*!isprime(d)) - 1; \\ Michel Marcus, Jun 12 2019

Crossrefs

Cf. A000203, A035322, A035321, A060278, A023890.

Cf. A074372.

Sequence in context: A380490 A179939 A163407 * A075091 A132953 A387470

Adjacent sequences: A023888 A023889 A023890 * A023892 A023893 A023894

Keyword

nonn

Author

Olivier Gérard

Status

approved