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Search: id:A049221
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| A049221 |
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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. |
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+0
2
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| 1, 0, 1, 4, 14, 46, 148, 474, 1518, 4864, 15590, 49974, 160196, 513522, 1646134, 5276800, 16915150, 54222686, 173814580, 557174698, 1786062174, 5725346304, 18352995094, 58831800038, 188589419748, 604536478850, 1937883656166
(list; graph; listen)
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OFFSET
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1,4
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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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Hickerson reference.
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FORMULA
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G.f.: x (1-x)^2 (1-3x+x^2)/(1-5x+7x^2-4x^3)
a(n) = 5a(n-1) - 7a(n-2) + 4a(n-3) for n >= 6
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MATHEMATICA
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a[ n_ ] := a[ n ]=If[ n<6, {1, 0, 1, 4, 14}[ [ n ] ], 5a[ n-1 ]-7a[ n-2 ]+4a[ n-3 ] ]
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CROSSREFS
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a(n) = A049219(n-1) + A049222(n) for n >= 3
Sequence in context: A030267 A026290 A027649 this_sequence A081670 A124805 A121530
Adjacent sequences: A049218 A049219 A049220 this_sequence A049222 A049223 A049224
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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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