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 A247744 #10 Feb 06 2017 18:37:00
%S A247744 0,0,0,1,8,76,752,7728,76138,683740,6016612,52483260,453461612,
%T A247744 3873299197,32718505462,274368261226,2287565667766,18973561019204,
%U A247744 156622182476404,1287479429817295,10545476231546336,86101164740284870,700977394316933694,5692064258367318333
%N A247744 Total number of X shapes in all tilings of a 5 X n rectangle with pentominoes of any shape.
%H A247744 Alois P. Heinz, <a href="/A247744/b247744.txt">Table of n, a(n) for n = 0..500</a>
%H A247744 Wikipedia, <a href="https://en.wikipedia.org/wiki/Pentomino">Pentomino</a>
%F A247744 a(n) = Sum_{k>0} k * A247711(n,k).
%Y A247744 Cf. A174249, A233427, A247711.
%K A247744 nonn
%O A247744 0,5
%A A247744 _Alois P. Heinz_, Sep 23 2014