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%I #17 Mar 10 2019 13:04:59
%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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