A123801 Number of ways to build a contiguous building with n LEGO blocks of size 1 X 6 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, 31, 76, 365, 920, 4691, 12695, 65708, 184458, 967195, 2798734, 14837523, 43829052, 234029147, 703643181

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.

M. Abrahamsen and S. Eilers, On the asymptotic enumeration of LEGO structures, Exper Math. 20 (2) (2011) 145-152.

B. Durhuus and S. Eilers, On the entropy of LEGO, arXiv:math.CO/0504039

Crossrefs

Sequence in context: A080871 A358586 A102666 * A359706 A156228 A218959

Adjacent sequences: A123798 A123799 A123800 * A123802 A123803 A123804

Keyword

nonn

Author

Søren Eilers, Oct 29 2006

Extensions

a(12)-a(17) from Søren Eilers, Sep 16 2018

Status

approved