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.

1, 3, 5, 18, 42, 170, 405, 1769, 4507, 20081, 53008, 240428, 652160, 3002052, 8299727, 38570944, 108387118, 507729121, 1444491686, 6809942459

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(12)-a(20) from Søren Eilers, Sep 16 2018