3
$\begingroup$

Let $\mathcal{P}$ be a polyomino. Two or more rooks on $\mathcal{P}$ are called non-attacking if no path of edge-adjacent cells of $\mathcal{P}$ connects any pair of them along a row or a column.

Let $F_1,F_2$ be two arrangements of $k$ non-attacking rooks.

$$ \begin{aligned} F_1 \sim F_2 \quad &\text{if } F_2 \text{ can be obtained from } F_1 \text{ by moving two or more rooks from}\\ &\text{diagonal to anti-diagonal positions, or vice versa, within $\mathcal{P}$.} \end{aligned} $$

Let $\tilde{r}(k,\mathcal{P})$ denote the number of equivalence classes of the arrangements of $k$ non-attacking rooks. The maximum number of non-attacking rooks that can be placed on a polyomino $\mathcal{P}$ is called rook number of $\mathcal{P}$, denoted by $r(\mathcal{P})$.

We can define the following polynomial:
$$ \tilde{r}_{\mathcal{P}}(t) = \sum_{k=0}^{r(\mathcal{P})} \tilde{r}(k,\mathcal{P})t^k $$

The figure below illustrates four $3$-rook configurations in $\mathcal{P}$ that are equivalent under switches.

The rook number of $\mathcal{P}$ is four, and $ \tilde{r}_{\mathcal{P}}(t) = 1 + 12t + 30t^2 + 16t^3 + t^4. $

Question. It is well known that the classical rook polynomial is related to graph theory, as it coincides with the matching polynomial of a bipartite graph. My question is whether the variant of the rook polynomial described above is also related to graph theory.

Motivation behind the question. I encountered this variant of the rook polynomial in a context related to Combinatorial Commutative Algebra, where it provides a combinatorial interpretation of the Hilbert series of certain binomial ideals associated with polyominoes, namely the determinantal ideals generated by inner $2$-minors of matrices that can be represented as polyominoes. You can see

Jahangir, Rizwan; Navarra, Francesco, Shellable simplicial complex and switching rook polynomial of frame polyominoes, J. Pure Appl. Algebra 228, No. 6, Article ID 107576, 25 p. (2024). ZBL1533.05053.

and the references there in. Given this, I also wondered whether this version of the rook polynomial might admit connections with graph theory, in analogy with the classical rook polynomial.

$\endgroup$

1 Answer 1

3
$\begingroup$

Certainly the operation you describe has been studied by graph theorists. For example, The Perfect Matching Reconfiguration Problem by Bonamy et al. studies the computational complexity of determining whether two given perfect matchings are equivalent.

Whether your modified rook polynomial has been studied before, I don't know. At first glance, it looks to me like an intractable object about which not much can be said; if so, then maybe nobody has proved anything about it worth publishing. If you have a specific theorem or conjecture in mind then that might make it easier to search for related literature.

$\endgroup$
3
  • $\begingroup$ Thank you for your comment! I saw this version of "rook polynomial" in a context related to combinatorial commutative algebra as it provides a combinatorial interpretation of the Hilbert series of binomial ideals associated to polyominoes, which are the determinantal ideals generated by inner 2-minors of matrices that can be represented as polyominoes (sciencedirect.com/science/article/pii/S002240492300258X and the references there in). I wonder that maybe it had a connection with graph theory, as the standard rook polynomial $\endgroup$ Commented Aug 17 at 13:37
  • 2
    $\begingroup$ @Chess I see. You might consider editing your original question to explain all this. It will improve the quality of your question. MO readers generally like to see the motivation behind the question. $\endgroup$ Commented Aug 18 at 13:38
  • 1
    $\begingroup$ I followed your suggestion and I added the motivation behind the question. Thank you very much! $\endgroup$ Commented Aug 18 at 13:49

You must log in to answer this question.

Start asking to get answers

Find the answer to your question by asking.

Ask question

Explore related questions

See similar questions with these tags.