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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. 0
1, 4, 7, 34, 85, 461, 1175, 6871, 18807, 112406, 318571, 1936113, 5653062, 34848639, 103771770, 645701202, 1955873614 (list; graph; refs; listen; history; text; internal format)
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

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Last modified May 16 00:49 EDT 2021. Contains 343937 sequences. (Running on oeis4.)