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 A219868 #9 Dec 07 2012 12:45:27 %S A219868 1,2,15,143,1069,9612,82634,707903,6155230,53074167,458901955, %T A219868 3968548571,34293794622,296497719200,2563023778341,22155553744380, %U A219868 191528023181443,1655649630655481,14312296720331975,123722938782920490,1069522524403416934,9245501816227527991 %N A219868 Number of tilings of a 5 X n rectangle using dominoes and straight (3 X 1) trominoes. %H A219868 Alois P. Heinz, <a href="/A219868/b219868.txt">Table of n, a(n) for n = 0..500</a> %e A219868 a(2) = 15, because there are 15 tilings of a 5 X 2 rectangle using dominoes and straight (3 X 1) trominoes: %e A219868 .___. .___. .___. .___. .___. .___. .___. .___. %e A219868 | | | |___| | | | |___| | | | |___| | | | |___| %e A219868 | | | |___| |_|_| | | | | | | |___| |_|_| | | | %e A219868 |_|_| |___| |___| |_|_| |_|_| |___| |___| |_|_| %e A219868 | | | | | | | | | | | | |___| |___| |___| |___| %e A219868 |_|_| |_|_| |_|_| |_|_| |___| |___| |___| |___| %e A219868 .___. .___. .___. .___. ._._. ._._. .___. %e A219868 | | | | | | |___| |___| | | | | | | |___| %e A219868 |_|_| |_|_| |___| |___| |_| | | |_| | | | %e A219868 | | | | | | | | | | | | | |_| |_| | | | | %e A219868 | | | |_|_| | | | |_|_| | | | | | | |_|_| %e A219868 |_|_| |___| |_|_| |___| |_|_| |_|_| |___| %Y A219868 Column k=5 of A219866. %K A219868 nonn %O A219868 0,2 %A A219868 _Alois P. Heinz_, Nov 30 2012