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%I #13 Sep 16 2018 09:56:29
%S 1,4,7,31,76,365,920,4691,12695,65708,184458,967195,2798734,14837523,
%T 43829052,234029147,703643181
%N Number of ways to build a contiguous building with n LEGO blocks of size 1 X 6 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.CO/0504039
%K nonn
%O 1,2
%A _Søren Eilers_, Oct 29 2006
%E a(12)-a(17) from _Søren Eilers_, Sep 16 2018
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