

A123769


Number of ways to build a contiguous building with n LEGO blocks of size 1 X 2 on top of a fixed block of the same size so that the building is flat, i.e., with all blocks in parallel position and symmetric after a rotation by 180 degrees.


0



1, 2, 3, 6, 11, 23, 43, 93, 181, 392, 783, 1710, 3467, 7623, 15655, 34543, 71688, 158721, 332006, 737213, 1552365, 3455004, 7315948, 16315297, 34711474, 77544359, 165653643, 370623055
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OFFSET

1,2


COMMENTS

The base block is not counted among the n and must be the only block in the bottom layer of the building.


LINKS



CROSSREFS



KEYWORD

nonn,more


AUTHOR



EXTENSIONS



STATUS

approved



