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A123776
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Number of ways to build a contiguous building with n LEGO blocks of size 1 X 3 on top of a fixed block of the same size so that the building is flat, i.e., with all blocks in parallel position.
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0
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5, 28, 185, 1287, 9456, 71167, 549076, 4300975, 34154107, 273998400, 2217436152, 18074303774, 148232462372
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OFFSET
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1,1
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COMMENTS
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The base block is not counted among the n and must be the only block in the bottom layer of the building.
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LINKS
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CROSSREFS
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KEYWORD
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nonn,hard
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AUTHOR
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STATUS
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approved
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