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A073255 Sum of divisors of n-th composite number. 12

%I

%S 7,12,15,13,18,28,24,24,31,39,42,32,36,60,31,42,40,56,72,63,48,54,48,

%T 91,60,56,90,96,84,78,72,124,57,93,72,98,120,72,120,80,90,168,96,104,

%U 127,84,144,126,96,144,195,114,124,140,96,168,186,121,126,224,108,132

%N Sum of divisors of n-th composite number.

%H Robert Israel, <a href="/A073255/b073255.txt">Table of n, a(n) for n = 1..10000</a>

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

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

%p map(numtheory:-sigma, remove(isprime,[$4..100])); # _Robert Israel_, Aug 27 2018

%t c[x_] := FixedPoint[x+PrimePi[ # ]+1&, x] Table[DivisorSigma[1, c[w]], {w, 1, 128}]

%t DivisorSigma[1,#]&/@(Select[Range[100],CompositeQ]) (* Requires Mathematica version 10 or later *) (* _Harvey P. Dale_, Mar 10 2019 *)

%o (PARI) lista(nn) = forcomposite(c=1, nn, print1(sigma(c), ", ")); \\ _Michel Marcus_, Feb 21 2016

%Y Cf. A000203, A002808, A035004.

%K nonn

%O 1,1

%A _Labos Elemer_, Jul 22 2002

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Last modified May 12 06:54 EDT 2021. Contains 343820 sequences. (Running on oeis4.)