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 A219972 #9 Nov 28 2016 00:37:44 %S A219972 1,0,4,19,70,456,2530,12343,66653,372429,1910822,10162059,55583124, %T A219972 291195073,1550518247,8363883923,44269140576,235940940726, %U A219972 1264437357202,6723179849519,35846240541026,191571690239220,1020536749613565,5442225134864810,29051213879574079 %N A219972 Number of tilings of an 8 X n rectangle using straight (3 X 1) trominoes and 2 X 2 tiles. %H A219972 Alois P. Heinz, <a href="/A219972/b219972.txt">Table of n, a(n) for n = 0..100</a> %e A219972 a(2) = 4, because there are 4 tilings of an 8 X 2 rectangle using straight (3 X 1) trominoes and 2 X 2 tiles: %e A219972 .___. .___. .___. .___. %e A219972 | | | | | | | | | | %e A219972 |___| |___| | | | | | | %e A219972 | | | | | |_|_| |_|_| %e A219972 |___| | | | | | | | | %e A219972 | | |_|_| |___| | | | %e A219972 |___| | | | | | | |_|_| %e A219972 | | | | | | | | | | %e A219972 |___| |_|_| |_|_| |___| %Y A219972 Column k=8 of A219967. %K A219972 nonn %O A219972 0,3 %A A219972 _Alois P. Heinz_, Dec 02 2012