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 A219870 #8 Dec 07 2012 12:44:43 %S A219870 1,3,60,1562,29907,707903,15859323,354859954,8061851335,181382499259, %T A219870 4095897476480,92476840837163,2086314577400136,47096964973772265, %U A219870 1062921614745008697,23989328157229264043,541446343762904191567,12220135872229640539724 %N A219870 Number of tilings of a 7 X n rectangle using dominoes and straight (3 X 1) trominoes. %H A219870 Alois P. Heinz, <a href="/A219870/b219870.txt">Table of n, a(n) for n = 0..250</a> %e A219870 a(1) = 3, because there are 3 tilings of a 7 X 1 rectangle using dominoes and straight (3 X 1) trominoes: %e A219870 ._. ._. ._. %e A219870 | | | | | | %e A219870 | | |_| |_| %e A219870 |_| | | | | %e A219870 | | | | |_| %e A219870 |_| |_| | | %e A219870 | | | | | | %e A219870 |_| |_| |_| %Y A219870 Column k=7 of A219866. %K A219870 nonn %O A219870 0,2 %A A219870 _Alois P. Heinz_, Nov 30 2012