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%I #25 May 11 2019 11:48:28
%S 1,1,2,6,3,15,15,105,105,105,105,1155,1155,15015,15015,15015,15015,
%T 255255,255255,4849845,4849845,4849845,4849845,111546435,111546435,
%U 111546435,111546435,111546435,111546435,3234846615,3234846615,100280245065,100280245065
%N a(n) is the denominator of Sum_{i=1..n} i!/(i^2).
%F Denominator of Sum_{i=1..n} A000142(i)/A000290(i).
%e n=0: the empty sum is 0 by convention, so a(0)=1.
%e n=1: the sum is 1, so a(1)=1.
%e n=2: the sum is 1 + 1/2 = 3/2 so a(2) = 2.
%t Join[{1},Accumulate[Table[n!/n^2,{n,40}]]//Denominator] (* _Harvey P. Dale_, May 11 2019 *)
%o (PARI) a(n) = denominator(sum(i=1, n, i!/i^2)); \\ _Michel Marcus_, Mar 18 2018
%Y Cf. A121565 (numerator).
%K easy,frac,nonn
%O 0,3
%A _Jonathan Vos Post_, Aug 07 2006
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