A123776 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.

5, 28, 185, 1287, 9456, 71167, 549076, 4300975, 34154107, 273998400, 2217436152, 18074303774, 148232462372

Offset: 1

Author

Søren Eilers, Oct 29 2006

Keywords

• nonn
• hard

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.