A123817 Number of ways to build a contiguous building with n LEGO blocks of size 1 X 8 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.

1, 5, 9, 51, 131, 807, 2129, 13967, 39701, 265152, 781478, 5287858, 16094282, 110147280, 342365511, 2360643691

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.

Links

• Index entry for sequences related to LEGO blocks

Crossrefs

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

Extensions

a(11)-a(16) from Søren Eilers, Sep 16 2018