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Number of ways to build a contiguous building with n LEGO blocks of size 3 X 3 on top of a fixed block of the same size so that the building is symmetric after a rotation by 180 degrees.
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%I #8 Oct 09 2013 14:20:53

%S 1,9,17,186,523,6478,19077,247945,796692

%N Number of ways to build a contiguous building with n LEGO blocks of size 3 X 3 on top of a fixed block of the same size so that the building is 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