A123784 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.

7, 55, 511, 5000, 51674, 547204, 5940045, 65472189, 731569499, 8258444996, 94045306573

Offset

1,1

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..11.

Crossrefs

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

Sequence in context: A225032 A124403 A005012 * A091695 A002882 A094905

Adjacent sequences: A123781 A123782 A123783 * A123785 A123786 A123787

Keyword

nonn,hard

Author

Søren Eilers, Oct 29 2006

Status

approved