A123783 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 symmetric after a rotation by 180 degrees.

1, 11, 21, 248, 712, 9182, 28297, 375483, 1266013

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..9.

Crossrefs

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

Sequence in context: A083177 A110466 A110383 * A163288 A092806 A138485

Adjacent sequences: A123780 A123781 A123782 * A123784 A123785 A123786

Keyword

nonn,more

Author

Søren Eilers, Oct 29 2006

Status

approved