A073255 Sum of divisors of n-th composite number.

7, 12, 15, 13, 18, 28, 24, 24, 31, 39, 42, 32, 36, 60, 31, 42, 40, 56, 72, 63, 48, 54, 48, 91, 60, 56, 90, 96, 84, 78, 72, 124, 57, 93, 72, 98, 120, 72, 120, 80, 90, 168, 96, 104, 127, 84, 144, 126, 96, 144, 195, 114, 124, 140, 96, 168, 186, 121, 126, 224, 108, 132

Offset: 1

Author

Labos Elemer, Jul 22 2002

Keywords

• nonn

Examples

			First composite is 4, sigma[4]=1+2+4=7=a(1).
		

Links

• Robert Israel, Table of n, a(n) for n = 1..10000

Crossrefs

Cf. A000203, A002808, A035004.

Programs

Maple

map(numtheory:-sigma, remove(isprime,[$4..100])); # Robert Israel, Aug 27 2018

Mathematica

c[x_] := FixedPoint[x+PrimePi[ # ]+1&, x] Table[DivisorSigma[1, c[w]], {w, 1, 128}]
DivisorSigma[1,#]&/@(Select[Range[100],CompositeQ]) (* Requires Mathematica version 10 or later *) (* Harvey P. Dale, Mar 10 2019 *)

PARI

lista(nn) = forcomposite(c=1, nn, print1(sigma(c), ", ")); \\ Michel Marcus, Feb 21 2016

Formula

a(n) = A000203(A002808(n)).