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 A278876 #8 Feb 06 2017 12:46:48 %S A278876 1,1,25,311,4101,56864,728857,9616962,127040707,1669454156, %T A278876 22011707486,289906000417,3817261693883,50281947199556, %U A278876 662232832054368,8721984517729672,114875834939622917,1512983475914685706,19927016855891011234,262452496836729312910 %N A278876 Number of tilings of a 4 X n rectangle using pentominoes of any shape and monominoes. %H A278876 Alois P. Heinz, <a href="/A278876/b278876.txt">Table of n, a(n) for n = 0..893</a> %H A278876 Wikipedia, <a href="https://en.wikipedia.org/wiki/Pentomino">Pentomino</a> %e A278876 a(2) = 25: %e A278876 .___. .___. .___. .___. .___. .___. .___. .___. %e A278876 | ._| |_|_| |_. | |_|_| | |_| |_| | | ._| |_. | %e A278876 | |_| | ._| |_| | |_. | | |_| |_| | | |_| |_| | %e A278876 |___| | |_| |___| |_| | | |_| |_| | | |_| |_| | %e A278876 |_|_| |___| |_|_| |___| |___| |___| |_|_| |_|_| %e A278876 .___. .___. .___. .___. .___. .___. .___. .___. %e A278876 | | |_|_| | | |_|_| | |_| |_|_| |_| | |_|_| %e A278876 | ._| | | |_. | | | | | | |_| | | |_| | %e A278876 |_|_| | ._| |_|_| |_. | |___| | | |___| | | %e A278876 |_|_| |_|_| |_|_| |_|_| |_|_| |___| |_|_| |___| %e A278876 .___. .___. .___. .___. .___. .___. .___. .___. .___. %e A278876 | |_| |_| | | |_| |_| | | |_| |_| | |_| | | |_| |_|_| %e A278876 | |_| |_| | | ._| |_. | | |_| |_| | | ._| |_. | |_|_| %e A278876 | ._| |_. | | |_| |_| | |_. | | ._| | |_| |_| | |_|_| %e A278876 |_|_| |_|_| |_|_| |_|_| |_|_| |_|_| |_|_| |_|_| |_|_| . %Y A278876 Column k=4 of A278657. %K A278876 nonn %O A278876 0,3 %A A278876 _Alois P. Heinz_, Nov 29 2016