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 A049219 #16 Jul 02 2023 14:25:53
%S A049219 1,1,3,10,33,107,344,1103,3535,11330,36317,116415,373176,1196243,
%T A049219 3834643,12292218,39403561,126310851,404898200,1297929287,4160602439,
%U A049219 13337099986,42753000005,137047709879,439315949304,1408257777387
%N A049219 Number of horizontally convex n-ominoes in which the top row has exactly 1 square.
%H A049219 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 A049219 <a href="/index/Rec#order_03">Index entries for linear recurrences with constant coefficients</a>, signature (5, -7, 4).
%F A049219 G.f.: x (1-x)^2 (1-2x)/(1-5x+7x^2-4x^3).
%F A049219 a(n) = 5a(n-1) - 7a(n-2) + 4a(n-3) for n >= 5.
%F A049219 a(n) = A001169(n-1) + A049221(n) for n >= 2.
%t A049219 a[ n_ ] := a[ n ]=If[ n<5, {1, 1, 3, 10}[ [ n ] ], 5a[ n-1 ]-7a[ n-2 ]+4a[ n-3 ] ]
%t A049219 LinearRecurrence[{5,-7,4},{1,1,3,10,33,107},40] (* _Harvey P. Dale_, Nov 19 2019 *)
%Y A049219 Cf. A001169, A049221.
%K A049219 nonn,easy
%O A049219 1,3
%A A049219 _Dean Hickerson_, Aug 10 1999