A123785 Number of ways to build a contiguous building with n LEGO blocks of size 1 X 4 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, 16, 36, 124, 287, 1043, 2573, 9448, 24161, 89827, 235902, 886047, 2372494, 8968829, 24410414, 92783928, 255723571, 976331352

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