A023891 - OEIS

%I #19 Jun 20 2022 04:21:32

%S 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,

%T 46,0,61,0,60,33,34,35,85,0,38,39,82,0,83,0,70,69,46,0,118,49,85,51,

%U 82,0,114,55,110,57,58,0,157,0,62,93,124,65,127,0,106,69,129,0

%N Sum of composite divisors of n.

%H Vincenzo Librandi, <a href="/A023891/b023891.txt">Table of n, a(n) for n = 1..1000</a>

%F a(n) = A023890(n) - 1. - _Sean A. Irvine_, Jun 11 2019

%t Array[ Plus @@ (Select[ Divisors[ # ], (!PrimeQ[ # ] && #>1)& ])&, 75 ]

%t a[n_] := DivisorSigma[1, n] - Plus @@ FactorInteger[n][[;; , 1]] - 1; a[1] = 0; Array[a, 100] (* _Amiram Eldar_, Jun 20 2022 *)

%o (PARI) a(n) = sumdiv(n, d, d*!isprime(d)) - 1; \\ _Michel Marcus_, Jun 12 2019

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

%K nonn

%O 1,4

%A _Olivier Gérard_