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 A281433 #22 May 09 2021 17:35:37
%S A281433 1,1,2,10,40,296,1576,15352,104000,1276480,10556416,156843776,
%T A281433 1533722752,26777626240,302395339520,6068829396736,77740741758976,
%U A281433 1763457842941952,25267740818452480,639308368122204160,10131932297407840256,282891828731667890176
%N A281433 Number of maximal matchings in the 2 X n rook graph.
%H A281433 Andrew Howroyd, <a href="/A281433/b281433.txt">Table of n, a(n) for n = 0..100</a> (corrected by Ray Chandler, Jan 19 2019)
%F A281433 a(n) = Sum_{k=0..n/2} (2*k-1)!!^2 * binomial(n,2*k) * (1 + 2*k*(n-2*k)).
%t A281433 a[n_] := Sum[(2*k-1)!!^2 * Binomial[n, 2*k] * (1 + 2*k*(n-2*k)), {k, 0, n/2} ]; Table[a[n], {n, 0, 21}] (* _Jean-François Alcover_, Nov 01 2017, after _Andrew Howroyd_ *)
%o A281433 (PARI) a(n) = sum(k=0, n\2, ((2*k)!/(2^k*k!))^2 * binomial(n,2*k) * (1 + 2*k*(n-2*k)));
%Y A281433 Row n=2 of A341847.
%Y A281433 Cf. A081919 (perfect matchings), A270229, A289198.
%K A281433 nonn
%O A281433 0,3
%A A281433 _Andrew Howroyd_, Oct 05 2017