A123793 - OEIS

A123793

Number of ways to build a contiguous building with n LEGO blocks of size 1 X 5 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.

%I #12 Sep 16 2018 09:56:33

%S 1,3,5,18,42,170,405,1769,4507,20081,53008,240428,652160,3002052,

%T 8299727,38570944,108387118,507729121,1444491686,6809942459

%N Number of ways to build a contiguous building with n LEGO blocks of size 1 X 5 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(12)-a(20) from _Søren Eilers_, Sep 16 2018