# Have you seen my matroid?

Let $M(n,k)$ be the matroid on the ground set $\{\pm 1,\ldots,\pm n\}$ for which a set is independent if and only if it contains at most $k$ pairs $\pm i$. Note that the signed permutation group (the Coxeter group of type $B_n$) acts on this matroid. Questions:

1. Does this matroid have a name?
2. Has it been studied before?
3. Is there a nice formula for its characteristic polynomial?

Here are some boring special cases:

• $M(n,n)$ is the Boolean matroid on $2n$ elements.

• $M(n,n-1)$ is the uniform matroid of rank $2n-1$ on $2n$ elements.

• $M(n,0)$ is the direct sum of $n$ copies of the uniform matroid of rank 1 on 2 elements.

The first interesting case is $M(3,1)$, which has rank 4 and characteristic polynomial

$$q^4 - 6q^3 + 15q^2 - 17q + 7$$

I am also interested in truncations of this matroid. That is, let $M(n,k,d)$ be the matroid on the ground set $\{\pm 1,\ldots,\pm n\}$ for which a set is independent if and only if it contains at most $k$ pairs $\pm i$ and has size at most $d$. All of the same questions apply!

Remark: I would like to regard these matroids as type B analogues of uniform matroids. Uniform matroids are the permutation-invariant matroids on the ground set $\{1,\ldots,n\}$, while these are the signed-permutation-invariant matroids on the ground set $\{\pm 1,\ldots,\pm n\}$.

• I hate to be a spoilsport but the title, while cute and attention-grabbing, is a bit too vague for my tastes. As it stands, a reader seeing the title on the main page or a reader that might see the title in the "Related" list on another question page will have no idea of what "your matroid" is like.
– j.c.
Oct 30 '17 at 21:14
• @j.c.: Well, a quick search shows 47 questions with titles of the form "Does this ___ have a name?" Your criticisms would seem to apply to most of those titles as well. It doesn't seem quite obvious how to produce a more suitable title in such cases. (For that matter, "Does this matroid have a name?" was already taken.) Oct 31 '17 at 0:22
• Let us hope the answering post is titled "How I Met Your Matroid". Gerhard "Sometimes Just Can't Stop Himself" Paseman, 2017.10.30. Oct 31 '17 at 0:44
• @Will Jagy - I was thinking more of goodreads.com/book/show/18209408-have-you-seen-my-dragon Oct 31 '17 at 1:30
• @LouisDeaett Yes, it's not obvious at all, but still I think it's important! Every question with a vague title is a question that could be improved to be that much more helpful to someone searching for answers to the same question. Anyways, I won't belabor the point. I voted the question up by the way.
– j.c.
Oct 31 '17 at 2:30

Let $U$ be the uniform matroid of rank $k$ on $n$. Since $U$ is orientable one can consider the Lawrence oriented matroid $\Lambda(U)$ associated with any orientation of $U$ (the Lawrence construction doesn't care about which orientation you take). Then $M(n,k)$ is precisely the underlying unoriented matroid $\underline{\Lambda(U)}$ of $\Lambda(U)$.
Also, the dual matroid $M^*(n,k)$ is a symplectic matroid, which explains why the group $B_n$ acts on the primal.
• $M(n,k)$ can be constructed by starting with the uniform matroid $U_{k,n}$, and then replacing each element with a series pair. Alternatively, $M^{*}(n,k)$ is constructed by starting with the uniform matroid $U_{n-k,n}$ and adding an element in parallel to each point of the uniform matroid. I'm not aware of any name for these classes in the matroid literature. Nov 7 '17 at 20:57
One can use Whitney's theorem to show that the characteristic polynomial is $$\sum_{i=0}^k{n\choose i}q^{k-i}(q-2)^{n-i} + \sum_{i=k+1}^n{n\choose i}(q-2)^{n-i}.$$ I doubt that this can be simplified.