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 A049220 #14 Jul 02 2023 14:26:59
%S A049220 0,0,1,3,9,28,89,285,914,2931,9397,30124,96565,309545,992266,3180775,
%T A049220 10196193,32684604,104772769,335856389,1076610978,3451151243,
%U A049220 11062904925,35462909836,113678819677,364405349233,1168126647770
%N A049220 Number of horizontally convex n-ominoes in which the top row has at least 2 squares and the rightmost square in the top row is above the leftmost square in the second row.
%H A049220 Dean Hickerson, <a href="http://www.cs.uwaterloo.ca/journals/JIS/HICK2/chcp.html">Counting Horizontally Convex Polyominoes</a>, J. Integer Sequences, Vol. 2 (1999), #99.1.8.
%H A049220 <a href="/index/Rec#order_03">Index entries for linear recurrences with constant coefficients</a>, signature (5, -7, 4).
%F A049220 G.f.: x^3 (1-x)^2/(1-5x+7x^2-4x^3).
%F A049220 a(n) = 5a(n-1) - 7a(n-2) + 4a(n-3) for n >= 6.
%F A049220 a(n) = a(n-1) + A001169(n-2) for n >= 3.
%t A049220 a[ n_ ] := a[ n ]=If[ n<6, {0, 0, 1, 3, 9}[ [ n ] ], 5a[ n-1 ]-7a[ n-2 ]+4a[ n-3 ] ]
%Y A049220 Cf. A001169.
%K A049220 nonn,easy
%O A049220 1,4
%A A049220 _Dean Hickerson_, Aug 10 1999