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 A071799 #20 Feb 16 2025 08:32:46
%S A071799 2,34,524,7970,121252,1850380,28337976,435443490,6711230900,
%T A071799 103711749284,1606464657096,24935144010764,387746052588104,
%U A071799 6039349005200440,94203136553911024,1471326505700038434,23007323485217888340,360154459563530689204,5643332975601670914600
%N A071799 Number of lattice paths in the lattice [0..2n] X [0..2n] which do not pass through the point (n,n).
%H A071799 Eric Weisstein's World of Mathematics, <a href="https://mathworld.wolfram.com/LatticePath.html">Lattice Path</a>
%F A071799 a(n) = binomial(4n, 2n) - binomial(2n, n)^2.
%F A071799 Also, a(n) = 2*Sum_{k=0..n-1} binomial(2n,k)^2. [_Dennis P. Walsh_, Mar 23 2012]
%p A071799 seq(2*sum(binomial(2*n,k)^2,k=0..(n-1)),n=1..20); # _Dennis P. Walsh_, Mar 23 2012
%t A071799 Table[Binomial[4n, 2n] - Binomial[2n, n]^2, {n, 1, 20}]
%Y A071799 Cf. A000984, A002894.
%K A071799 easy,nice,nonn
%O A071799 1,1
%A A071799 _T. D. Noe_, Jun 06 2002