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 A330658 #8 Mar 01 2020 20:45:14 %S A330658 1,1,1,23,1608,371500,328956227,1126022690953,14806761014366079, %T A330658 744494518032208898560,142687918961909575461700797, %U A330658 103997384424546478979311463191760,287748628673335369235353717267441839979,3018422667417995172483450578913829739500058615 %N A330658 Number of ways to place zero or more dominoes on an n X n grid with an equal number of horizontal and vertical dominoes. %e A330658 a(1) = a(2) = 1 because the only possibility is placing no dominoes at all. %e A330658 a(3) = 23 because there is 1 way to place no dominoes, 20 ways to place one horizontal domino and one vertical domino, and 2 ways to place two horizontal and two vertical dominoes. %Y A330658 Cf. A028420, A332714. %K A330658 nonn %O A330658 0,4 %A A330658 _Andrew Howroyd_, Mar 01 2020