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 A112100 #13 Jan 05 2025 19:51:38
%S A112100 1,2,12,5,280,2520,220,120120,144144,1361360,25865840,77597520,
%T A112100 22881320,371821450,11473347600,9242418900,6876359661600,
%U A112100 20629078984800,281488407200,118731810156960,254425307479200,8113340360725600,36090376087365600,9419588158802421600
%N A112100 Denominator of Sum_{i=1..n} 1/(i*C(2*i,i)).
%H A112100 C. Elsner, <a href="https://web.archive.org/web/2024*/https://www.fq.math.ca/Papers1/43-1/paper43-1-5.pdf">On recurrence formulas for sums involving binomial coefficients</a>, Fib. Q., 43,1 (2005), 31-45.
%F A112100 Sum_{i >= 1} 1/(i*C(2*i, i)) = Pi*sqrt(3)/9.
%e A112100 0, 1/2, 7/12, 3/5, 169/280, 1523/2520, 133/220, 72623/120120, 87149/144144, .... -> Pi*sqrt(3)/9.
%t A112100 Table[Sum[1/(i*Binomial[2i,i]),{i,n}],{n,0,30}]//Denominator (* _Harvey P. Dale_, May 11 2019 *)
%o A112100 (PARI) a(n) = denominator(sum(i=1, n, 1/(i*binomial(2*i, i)))); \\ _Michel Marcus_, Mar 10 2016
%Y A112100 Cf. A112099.
%K A112100 nonn,frac
%O A112100 0,2
%A A112100 _N. J. A. Sloane_, Nov 30 2005
%E A112100 Definition corrected by _Wolfdieter Lang_, Oct 07 2008