A123809 Number of ways to build a contiguous building with n LEGO blocks of size 1 X 7 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, 4, 7, 34, 85, 461, 1175, 6871, 18807, 112406, 318571, 1936113, 5653062, 34848639, 103771770, 645701202, 1955873614

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..17.

Crossrefs

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

Sequence in context: A143547 A149090 A103059 * A140981 A283819 A124624

Adjacent sequences: A123806 A123807 A123808 * A123810 A123811 A123812

Keyword

nonn

Author

Søren Eilers, Oct 29 2006

Extensions

a(11)-a(17) from Søren Eilers, Sep 16 2018

Status

approved