

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

Table of n, a(n) for n=1..28.
M. Abrahamsen and S. Eilers, On the asymptotic enumeration of LEGO structures, Exper Math. 20 (2) (2011) 145152.
B. Durhuus and S. Eilers, On the entropy of LEGO, arXiv:math/0504039 [math.CO], 2005.
S. Eilers, The LEGO counting problem, Amer. Math. Monthly, 123 (May 2016), 415426.
Index entry for sequences related to LEGO blocks


CROSSREFS

Sequence in context: A036653 A318031 A316500 * A093304 A036590 A247139
Adjacent sequences: A123766 A123767 A123768 * A123770 A123771 A123772


KEYWORD

nonn,more


AUTHOR

Søren Eilers, Oct 29 2006


EXTENSIONS

a(21)a(28) from Søren Eilers, Sep 16 2018


STATUS

approved



