A123775 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 symmetric after a rotation by 180 degrees.

2, 8, 24, 115, 451, 2474, 10377, 60048, 268402, 1600292, 7416480

Offset: 1

Author

Søren Eilers, Oct 29 2006

Keywords

• nonn

Comments

The base block is not counted among the n and must be the only block in the bottom layer of the building.

Crossrefs

Cf. A112389, A123762 - A123827, A123829 - A123849.