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 A219971 #7 Dec 07 2012 12:52:27 %S A219971 1,0,3,13,31,170,804,2744,12343,55657,214485,923990,4008013,16241904, %T A219971 68963484,293830787,1216393406,5135225641,21711233759,90692128053, %U A219971 381960417890,1609647053073,6749229887672,28396742081001,119506154362773,501879008939056 %N A219971 Number of tilings of a 7 X n rectangle using straight (3 X 1) trominoes and 2 X 2 tiles. %H A219971 Alois P. Heinz, <a href="/A219971/b219971.txt">Table of n, a(n) for n = 0..500</a> %e A219971 a(2) = 3, because there are 3 tilings of a 7 X 2 rectangle using straight (3 X 1) trominoes and 2 X 2 tiles: %e A219971 ._._. .___. .___. %e A219971 | | | | | | | %e A219971 | | | |___| |___| %e A219971 |_|_| | | | | | %e A219971 | | | | | |___| %e A219971 |___| |_|_| | | | %e A219971 | | | | | | | %e A219971 |___| |___| |_|_| %Y A219971 Column k=7 of A219967. %K A219971 nonn %O A219971 0,3 %A A219971 _Alois P. Heinz_, Dec 02 2012