A123817 Number of ways to build a contiguous building with n LEGO blocks of size 1 X 8 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, 5, 9, 51, 131, 807, 2129, 13967, 39701, 265152, 781478, 5287858, 16094282, 110147280, 342365511, 2360643691

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..16.

Index entry for sequences related to LEGO blocks

Crossrefs

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

Sequence in context: A173776 A289909 A000324 * A369155 A124421 A262918

Adjacent sequences: A123814 A123815 A123816 * A123818 A123819 A123820

Keyword

nonn

Author

Søren Eilers, Oct 29 2006

Extensions

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

Status

approved