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 A278874 #12 Feb 06 2017 12:35:42 %S A278874 1,1,1,7,25,50,155,508,1343,3800,11438,32525,92333,268766,774302, %T A278874 2216976,6392865,18425916,52958070,152425812,438973764,1263109849, %U A278874 3634965137,10463959311,30116734921,86675829307,249478723992,718056248229,2066658063664,5948257601097 %N A278874 Number of tilings of a 2 X n rectangle using pentominoes of any shape and monominoes. %H A278874 Alois P. Heinz, <a href="/A278874/b278874.txt">Table of n, a(n) for n = 0..1000</a> %H A278874 Wikipedia, <a href="https://en.wikipedia.org/wiki/Pentomino">Pentomino</a> %F A278874 G.f.: -(x^10 +x^8 -x^6 -6*x^5 -x^4 -5*x^3 +1) / (x^15 +x^13 -2*x^11 -11*x^10 -2*x^9 -10*x^8 +x^7 +9*x^6 +12*x^5 +8*x^4 +11*x^3 +x -1). %e A278874 a(3) = 7: %e A278874 ._____. ._____. ._____. ._____. ._____. ._____. ._____. %e A278874 |_|_|_| | |_| | | ._. | | ._| |_. | | |_| |_| | %e A278874 |_|_|_| |_____| |_|_|_| |___|_| |_|___| |_____| |_____| . %Y A278874 Column k=2 of A278657. %K A278874 nonn,easy %O A278874 0,4 %A A278874 _Alois P. Heinz_, Nov 29 2016