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A123777 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 flat, i.e., with all blocks in parallel position and symmetric after a rotation by 180 degrees. 0
1, 2, 3, 7, 14, 37, 76, 217, 473, 1374, 3104, 9196, 21240, 63929, 150294, 456703, 1089683, 3338624, 8055267, 24845592, 60511113, 187665038, 460638240 (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..23.

CROSSREFS

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

Sequence in context: A006660 A306844 A213906 * A245899 A246747 A090828

Adjacent sequences:  A123774 A123775 A123776 * A123778 A123779 A123780

KEYWORD

nonn

AUTHOR

Søren Eilers, Oct 29 2006

EXTENSIONS

a(16)-a(23) from Søren Eilers, Sep 16 2018

STATUS

approved

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Last modified June 5 11:06 EDT 2020. Contains 334840 sequences. (Running on oeis4.)