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%I #11 Sep 16 2018 09:56:41
%S 1,2,3,7,14,37,76,217,473,1374,3104,9196,21240,63929,150294,456703,
%T 1089683,3338624,8055267,24845592,60511113,187665038,460638240
%N Number of ways to build a contiguous building with n LEGO blocks of size 1 X 3 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.
%Y Cf. A112389, A123762 - A123827, A123829 - A123849.
%K nonn
%O 1,2
%A _Søren Eilers_, Oct 29 2006
%E a(16)-a(23) from _Søren Eilers_, Sep 16 2018
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