This is a front-end for the Online Encyclopedia of Integer Sequences, made by Christian Perfect. The idea is to provide OEIS entries in non-ancient HTML, and then to think about how they're presented visually. The source code is on GitHub.
%I A071968 #8 Nov 04 2013 05:30:47
%S A071968 1,1,3,45,105,1575,10395,315315,45045,6891885,72747675,160044885,
%T A071968 1003917915,21751554825,35137127025,2183521465125,4512611027925,
%U A071968 76714387474725,40613499251325,3172365552631275
%N A071968 Denominators of coefficients of expansion of arctan(x)^2 = x^2-2/3*x^4+23/45*x^6-44/105*x^8+563/1575*x^10-3254/10395*x^12+ ...
%t A071968 a[n_] := (-1)^(n+1)*Sum[1/(n*(2*k-1)), {k, 1, n}] // Denominator; Table[a[n], {n, 0, 19}] (* _Jean-François Alcover_, Nov 04 2013 *)
%Y A071968 Cf. A002428.
%K A071968 nonn,easy,frac
%O A071968 0,3
%A A071968 _N. J. A. Sloane_, Jun 17 2002