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Denominator of Sum_{k=1..n} 1/sigma(k).
4

%I #4 May 25 2012 14:32:48

%S 1,3,12,84,28,42,168,280,3640,32760,32760,32760,4680,1170,4680,145080,

%T 145080,145080,145080,1015560,4062240,4062240,4062240,4062240,4062240,

%U 4062240,4062240,4062240,4062240,4062240,2031120,2031120,1015560,3046680,6093360,6093360

%N Denominator of Sum_{k=1..n} 1/sigma(k).

%e 1, 4/3, 19/12, 145/84, 53/28, 83/42, 353/168, …

%p with(numtheory): a:=n->numer(sum(1/sigma(k), k=1..n)): seq(a(n), n=1..50);

%t Denominator[Table[Sum[1/DivisorSigma[1,k],{k,1,n}],{n,1,50}]]

%Y Cf. A000203, A212717.

%K nonn,frac

%O 1,2

%A _Michel Lagneau_, May 25 2012