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%I #8 Mar 30 2012 16:52:02
%S 1,6,1092,4218760,27765973216255750329906360,
%T 229254309739144896253372216696442967123093789661296276592384463520
%N Define a series of lists by L_1 = [1], L_{k+1} = [i+1, i^2+i+1 : i in L_k]; then a(n) = denominator of Sum (1/i : i in L_n).
%C Sum (1/i : i in L_n) converges to Pi/4 as n -> oo.
%D J. Borwein, D. Bailey and R. Girgensohn, Experimentation in Mathematics: Computational Paths to Discovery, Peters, Boston, 2004. See Sect. 1.3.
%e 1, 5/6, 877/1092, 3349607/4218760, 21942759935479332971926241/27765973216255750329906360, ...
%p M:=6; s1:={1}; n1[1]:=1;
%p for n from 2 to M do
%p s2:={};
%p for i in s1 do s2:={op(s2), i+1, i^2+i+1 }; od:
%p n1[n] := add(1/i, i in s2):
%p s1:=s2;
%p od:
%p s3:=[seq(n1[i],i=1..M)];
%Y Cf. A190350.
%K nonn,frac
%O 1,2
%A _N. J. A. Sloane_, May 09 2011
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