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 A100521 #9 Jul 07 2022 02:19:50
%S A100521 1,4,72,1200,19600,635040,25613280,82450368,9275666400,595703908800,
%T A100521 2048086772160,23459903026560,413676290035008,4419618483280000,
%U A100521 3221901874311120000,361282596839420256000,2630246784565779288000,9628029406360113091200,1310481780310126504080000
%N A100521 Denominator of Sum_{k=0..2*n} (-1)^k/binomial(2*n, k)^2.
%H A100521 G. C. Greubel, <a href="/A100521/b100521.txt">Table of n, a(n) for n = 0..675</a>
%F A100521 a(n) = denominator( Sum_{k=0..2*n} (-1)^k/binomial(2*n, k)^2 ).
%e A100521 1, 7/4, 137/72, 2341/1200, 38629/19600, 1257937/635040, 50881679/25613280, 164078209/82450368, 18480100619/9275666400, 1187779852639/595703908800, ... = A100520/A100521
%t A100521 Table[Denominator[Sum[(-1)^k/Binomial[2*n,k]^2, {k,0,2*n}]], {n,0,30}] (* _G. C. Greubel_, Jun 25 2022 *)
%o A100521 (Magma) [Denominator( (&+[(-1)^k/Binomial(2*n,k)^2: k in [0..2*n]]) ): n in [0..30]]; // _G. C. Greubel_, Jun 25 2022
%o A100521 (SageMath) [denominator(sum((-1)^k/binomial(2*n,k)^2 for k in (0..2*n))) for n in (0..30)] # _G. C. Greubel_, Jun 25 2022
%o A100521 (PARI) a(n) = denominator(sum(k=0, 2*n, (-1)^k/binomial(2*n, k)^2)); \\ _Michel Marcus_, Jun 25 2022
%Y A100521 Cf. A100520.
%K A100521 nonn,frac
%O A100521 0,2
%A A100521 _N. J. A. Sloane_, Nov 25 2004
%E A100521 Definition corrected by _Alexander Adamchuk_, May 11 2007