A123777 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 and symmetric after a rotation by 180 degrees.

1, 2, 3, 7, 14, 37, 76, 217, 473, 1374, 3104, 9196, 21240, 63929, 150294, 456703, 1089683, 3338624, 8055267, 24845592, 60511113, 187665038, 460638240

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(16)-a(23) from Søren Eilers, Sep 16 2018