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

1,2

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..20.

Crossrefs

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

Sequence in context: A143546 A069066 A011964 * A196420 A199676 A303684

Adjacent sequences: A123790 A123791 A123792 * A123794 A123795 A123796

Keyword

nonn

Author

Søren Eilers, Oct 29 2006

Extensions

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

Status

approved