# [seqfan] Re: Tatami

Richard J. Mathar mathar at mpia-hd.mpg.de
Thu Mar 24 17:06:42 CET 2016

```The formula in http://list.seqfan.eu/pipermail/seqfan/2016-February/016160.html
seems to be correct. I computed (by brute force enumeration with a C++ program)
the arrangements of tatami tilings of the 3 X (2k+1) floor, k>=0,
with one monomer at an arbitrary position and 3k+1 dimers. Arrangements
which are symmetry-related by rotations or flips are counted with
multiplicity. The numerical result is

2, 10, 18, 38, 72, 136, 250, 454, 814, 1446, 2548...

This seems to have the g.f. -2*(x-1)*(2*x^2+4*x+1) ) / (x^2+x-1)^2  .

Then we have in the more general setup the number of tatami tilings of
the (2m+1) X(2k+1) floor with one monomer at an arbitrary position
and 2*(2*m*k+m+k) dimers.  The result is symmetric in m <-> k and it
suffices to tabulate the counts as a triangle m>=0, 0<=k<=m:

1
2  10
3  18 10
4  38  8 10
5  72 18  4 x
6 136 24  x x x
7 250 32  x x x x

The second column (or row) is the sequence already shown above.
The first column are the integers because in the 1 X (2k+1) case
the monomer can be placed basically at any position which leaves
an even number of free spaces at both sides for the dimers.

Richard
```