A123785 Number of ways to build a contiguous building with n LEGO blocks of size 1 X 4 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, 3, 5, 16, 36, 124, 287, 1043, 2573, 9448, 24161, 89827, 235902, 886047, 2372494, 8968829, 24410414, 92783928, 255723571, 976331352

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..20.

Crossrefs

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

Sequence in context: A369930 A077551 A106588 * A121646 A226205 A300533

Adjacent sequences: A123782 A123783 A123784 * A123786 A123787 A123788

Keyword

nonn

Author

Søren Eilers, Oct 29 2006

Extensions

a(14)-a(20) from Søren Eilers, Sep 16 2018

Status

approved