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%I #23 Sep 16 2018 09:56:51
%S 1,2,3,6,11,23,43,93,181,392,783,1710,3467,7623,15655,34543,71688,
%T 158721,332006,737213,1552365,3455004,7315948,16315297,34711474,
%U 77544359,165653643,370623055
%N 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.
%C The base block is not counted among the n and must be the only block in the bottom layer of the building.
%H M. Abrahamsen and S. Eilers, <a href="http://dx.doi.org/10.1080/10586458.2011.564539">On the asymptotic enumeration of LEGO structures</a>, Exper Math. 20 (2) (2011) 145-152.
%H B. Durhuus and S. Eilers, <a href="http://arxiv.org/abs/math/0504039">On the entropy of LEGO</a>, arXiv:math/0504039 [math.CO], 2005.
%H S. Eilers, <a href="http://www.jstor.org/stable/10.4169/amer.math.monthly.123.5.415">The LEGO counting problem</a>, Amer. Math. Monthly, 123 (May 2016), 415-426.
%H <a href="/wiki/Index_to_OEIS:_Section_Lc#LEGO">Index entry for sequences related to LEGO blocks</a>
%K nonn,more
%O 1,2
%A _Søren Eilers_, Oct 29 2006
%E a(21)-a(28) from _Søren Eilers_, Sep 16 2018
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