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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.
3
0, 0, 1, 3, 9, 28, 89, 285, 914, 2931, 9397, 30124, 96565, 309545, 992266, 3180775, 10196193, 32684604, 104772769, 335856389, 1076610978, 3451151243, 11062904925, 35462909836, 113678819677, 364405349233, 1168126647770
OFFSET
1,4
LINKS
Dean Hickerson, Counting Horizontally Convex Polyominoes, J. Integer Sequences, Vol. 2 (1999), #99.1.8.
FORMULA
G.f.: x^3 (1-x)^2/(1-5x+7x^2-4x^3).
a(n) = 5a(n-1) - 7a(n-2) + 4a(n-3) for n >= 6.
a(n) = a(n-1) + A001169(n-2) for n >= 3.
MATHEMATICA
a[ n_ ] := a[ n ]=If[ n<6, {0, 0, 1, 3, 9}[ [ n ] ], 5a[ n-1 ]-7a[ n-2 ]+4a[ n-3 ] ]
CROSSREFS
Cf. A001169.
Sequence in context: A358092 A333504 A199104 * A094790 A007822 A094164
KEYWORD
nonn,easy
AUTHOR
Dean Hickerson, Aug 10 1999
STATUS
approved