%I A049222
%S A049222 0,0,0,1,4,13,41,130,415,1329,4260,13657,43781,140346,449891,1442157,
%T A049222 4622932,14819125,47503729,152276498,488132887,1564743865,5015895108,
%U A049222 16078800033,51541709869,165220529546,529625878779,1697752526549
%N A049222 Number of horizontally convex n-ominoes in which the top row has exactly 
               1 square, which is not above the rightmost square in the second row 
               and the rightmost square in the second row is above the leftmost 
               square in the third row.
%D A049222 Dean Hickerson, Counting Horizontally Convex Polyominoes, J. Integer 
               Sequences, Vol. 2 (1999), #99.1.8.
%H A049222 <a href="http://www.cs.uwaterloo.ca/journals/JIS/index.html">Hickerson 
               reference.</a>
%F A049222 G.f.: x^4 (1-x)/(1-5x+7x^2-4x^3)
%F A049222 a(n) = 5a(n-1) - 7a(n-2) + 4a(n-3) for n >= 6
%t A049222 a[ n_ ] := a[ n ]=If[ n<6, {0, 0, 0, 1, 4}[ [ n ] ], 5a[ n-1 ]-7a[ n-2 
               ]+4a[ n-3 ] ]
%Y A049222 a(n) = a(n-1) + A049220(n-1) for n >= 2
%Y A049222 Sequence in context: A077284 A070428 A052529 this_sequence A001453 A141364 
               A005002
%Y A049222 Adjacent sequences: A049219 A049220 A049221 this_sequence A049223 A049224 
               A049225
%K A049222 nonn,easy
%O A049222 1,5
%A A049222 Dean Hickerson (dean.hickerson(AT)yahoo.com), Aug 10 1999