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 A247680 #18 Feb 06 2017 18:31:18 %S A247680 1,1,3,5,21,82,249,688,1879,5690,17932,55271,164427,485348,1451110, %T A247680 4395114,13313135,40073992,120200822,360897368,1086543152,3274191643, %U A247680 9858847241,29657925485,89206237151,268435863317,808022052324,2432169981689,7319562671432 %N A247680 Number of tilings of a 5 X n rectangle using n pentominoes of shapes W, I, L, F. %H A247680 Alois P. Heinz, <a href="/A247680/b247680.txt">Table of n, a(n) for n = 0..1000</a> %H A247680 Vaclav Kotesovec, <a href="/A247680/a247680.txt">G.f. and the recurrence (of order 324)</a> %H A247680 Wikipedia, <a href="https://en.wikipedia.org/wiki/Pentomino">Pentomino</a> %F A247680 a(n) ~ c * d^n, where d = 3.009533036298033336764263169394953980849599088993157702490314631810945318907..., c = 0.29272000293879867768013500033525343337565088925220444775140709413075274... (1/d is the root of the denominator, see g.f.). - _Vaclav Kotesovec_, May 19 2015 %e A247680 a(3) = 5: %e A247680 ._____. ._____. ._____. ._____. ._____. %e A247680 | | | | | |_. | | ._| | | | ._| |_. | | %e A247680 | | | | | | | | | | | | | | | | | | | | %e A247680 | | | | | | | | | | | | | | | | | | | | %e A247680 | | | | | | |_| |_| | | | |_| | | |_| | %e A247680 |_|_|_| |_|___| |___|_| |_|___| |___|_|. %e A247680 a(4) = 21: %e A247680 ._______. ._______. %e A247680 |_. |_. | | ._| ._| %e A247680 | |_. | | | |_. | | %e A247680 |_. |_| | | | |_| | %e A247680 | |___|_| |_| ._| | %e A247680 |_______| |___|___| ... . %Y A247680 Cf. A174249, A233427, A234312, A234931, A247443, A249762, A257866. %K A247680 nonn %O A247680 0,3 %A A247680 _Alois P. Heinz_, Sep 22 2014