A123809 Number of ways to build a contiguous building with n LEGO blocks of size 1 X 7 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, 4, 7, 34, 85, 461, 1175, 6871, 18807, 112406, 318571, 1936113, 5653062, 34848639, 103771770, 645701202, 1955873614

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.

Extensions

a(11)-a(17) from Søren Eilers, Sep 16 2018