A123822 Number of ways to build a contiguous building with n LEGO blocks of size 2 X 2 on top of a fixed block of the same size so that the building is symmetric after a rotation by 180 degrees.

1, 5, 9, 44, 111, 576, 1541, 8199, 23750, 127654, 384817, 2083235

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..12.

Crossrefs

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

Sequence in context: A176967 A110421 A176751 * A226065 A321718 A304127

Adjacent sequences: A123819 A123820 A123821 * A123823 A123824 A123825

Keyword

nonn

Author

Søren Eilers, Oct 29 2006

Status

approved