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

1,2

Comments

The base block is not counted among the n and must be the only block in the bottom layer of the building.

Links

Table of n, a(n) for n=1..23.

Crossrefs

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

Sequence in context: A306844 A213906 A364593 * A245899 A246747 A090828

Adjacent sequences: A123774 A123775 A123776 * A123778 A123779 A123780

Keyword

nonn

Author

Søren Eilers, Oct 29 2006

Extensions

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

Status

approved