A123775 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 symmetric after a rotation by 180 degrees.

2, 8, 24, 115, 451, 2474, 10377, 60048, 268402, 1600292, 7416480

Offset

1,1

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..11.

Crossrefs

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

Sequence in context: A295583 A150669 A157005 * A052624 A272590 A361991

Adjacent sequences: A123772 A123773 A123774 * A123776 A123777 A123778

Keyword

nonn

Author

Søren Eilers, Oct 29 2006

Status

approved