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A049219
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Number of horizontally convex n-ominoes in which the top row has exactly 1 square.
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2
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1, 1, 3, 10, 33, 107, 344, 1103, 3535, 11330, 36317, 116415, 373176, 1196243, 3834643, 12292218, 39403561, 126310851, 404898200, 1297929287, 4160602439, 13337099986, 42753000005, 137047709879, 439315949304, 1408257777387
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OFFSET
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1,3
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REFERENCES
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Dean Hickerson, Counting Horizontally Convex Polyominoes, J. Integer Sequences, Vol. 2 (1999), #99.1.8.
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LINKS
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Table of n, a(n) for n=1..26.
Hickerson reference.
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FORMULA
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G.f.: x (1-x)^2 (1-2x)/(1-5x+7x^2-4x^3)
a(n) = 5a(n-1) - 7a(n-2) + 4a(n-3) for n >= 5
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MATHEMATICA
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a[ n_ ] := a[ n ]=If[ n<5, {1, 1, 3, 10}[ [ n ] ], 5a[ n-1 ]-7a[ n-2 ]+4a[ n-3 ] ]
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CROSSREFS
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a(n) = A001169(n-1) + A049221(n) for n >= 2
Sequence in context: A053156 A120897 A077825 * A126184 A060557 A018920
Adjacent sequences: A049216 A049217 A049218 * A049220 A049221 A049222
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KEYWORD
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nonn,easy
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AUTHOR
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Dean Hickerson (dean.hickerson(AT)yahoo.com), Aug 10 1999
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STATUS
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approved
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