Standard complexes of matroids and lattice paths

Motivated by Gr\"obner basis theory for finite point configurations, we define and study the class of"standard complexes"associated to a matroid. Standard complexes are certain subcomplexes of the independence complex that are invariant under matroid duality. For the lexicographic term order, the standard complexes satisfy a deletion-contraction-type recurrence. We explicitly determine the lexicographic standard complexes for lattice path matroids using classical bijective combinatorics.


Introduction
Matroids come with a rich enumerative theory. Mostly, this can be attributed to the deletion-contraction paradigm that is inherent to matroid theory and that culminates in the existence of the Tutte polynomial; see [Whi92,Ch. 6]. Other objects encoding valuable enumerative properties can be associated to matroids. Trivially, the collection of independent sets I(M) of a matroid M is a simplicial complex, whose number of faces of various dimensions and topological features give enumerative invariants of M. Far less trivial are Brylawski's broken circuit complexes [Bry77]. Built on ideas of Whitney, these are simplicial complexes associated to matroids with a totally ordered groundset. Their enumerative and topological properties explain many combinatorial characteristics of the underlying matroid [Whi92,Ch. 7]. Both complexes, the independence as well as the broken circuit complex, can be constructed from a deletion-contraction-type process. The goal of this paper is to define and study a new class of simplicial complexes associated to matroids and to showcase their combinatorial structure. Encoding and studying combinatorial objects by means of zero-dimensional ideals and Gröbner bases has a long history; see [Lov94,dL95,Rob98,ARS02,HR03,Lau07] for a non-exhaustive selection. In particular [Lau07] emphasizes the use of standard monomials but the point of view of simplicial complexes and matroids has been largely neglected. Let us highlight some of the interesting properties of standard complexes of matroids: For any term order , the standard complex S (M) is a subcomplex of the independence complex I(M) of M (Corollary 2.5) and its number of faces is precisely the number of bases of M (Corollary 2.3). Moreover, the standard complex is invariant under matroid duality, that is, S (M) = S (M * ), where M * is the matroid dual to M (Proposition 2.6).
For a nonempty matroid M, we write m(M) for the largest element in its groundset E ⊆ N, and we write S lex (M) for the standard complex with respect to the lexicographic term order such that x 1 ≻ x 2 ≻ · · · . Then the standard complex has the following deletion-contraction-type decomposition. Recall that for a simplicial complex K and v not a vertex of K, the cone of K with apex v is the complex v * K := K ∪ {v∪σ : σ ∈ K}.
Theorem 1.1. Let M be a matroid and m = m(M). If m is not a loop or coloop, then This gives a recursive definition of S lex (M).
Recall that for a map f : X → Y between topological spaces, the mapping cone is the topological space Y ⊔ f (X) cone(f (X)). Thus Theorem 1.1 states that S lex (M) is the mapping cone associated to the inclusion In Section 3, we prove Theorem 1.1 and recursively construct the bijection in Corollary 1.2 explicitly.
We call a subclass M ′ ⊆ M of matroids such that M\m(M), M/m(M) ∈ M ′ for every M ∈ M ′ max-minor-closed and observe that Corollary 1.2 provides a tool to understand the standard complex for matroids in the class M ′ by explicitly constructing a family {Λ M } M ∈M ′ of bijections and then showing that this family has the proposed properties.
In the second part of the paper, we implement this idea in the case of the (max-)minorclosed class of lattice path matroids. Lattice paths in Z 2 with only north or east steps are partially ordered by 'staying weakly below'. It was noted by Bonin, de Mier, and Noy [BdMN03] that the collection of lattice paths between two bounding paths U (for upper boundary) and L (for lower boundary) with common endpoints are the bases for a (transversal) matroid, called a lattice path matroid. The combinatorics of lattice path matroids (and lattice path in general) has been of considerable interest in recent years; see, for example, [Ard03,BdM06,Sch10,Sch11,ER16,KMSRA18]. It turns out that our construction of compatible bijections on the (max-)minor-closed class of lattice path matroids relates to classical bijective combinatorics. We explicitly describe the family Λ M in this case in Section 4; it can be nicely described in terms of lattice path combinatorics and is a well-known statistic in certain cases (Theorems 4.4 and 4.8). Surprisingly, we will observe that the bijection in the case of lattice path matroids does only depend on the lower boundary rather than on the complete set of bases in this case (Corollary 4.9). For trivial lower and upper boundaries, the standard complex in question appeared already in the literature and was discussed in [ARS02], see also [HR03, Thm. 1.2].
2. 0/1-configurations and standard complexes . , x n ] be the polynomial ring in n variables. A term order on the collection of monomials Mon n = {x α : α ∈ N n } is a total order such that 0 Every finite collection of points V ⊂ R n is an algebraic variety and the knowledge of Std (I(V )) is of the essence for example in polynomial optimization [Lau07] or the design of experiments [Rob98]. We refer the reader to the wonderful book [CLO15] for Gröbner basis theory in general.
i − x i vanishes on V for every i. Independent of the term order, we have x 2 i ∈ in (I(V )) which shows that Proposition 2.1. If V is a 0/1-point configuration, then Std (I(V )) is a collection of squarefree monomials.   i − x i for i = 1, . . . , n as well as x 1 + · · · + x n − r and x C for C ∈ C(M).
Proposition 2.4 yields the following result. Proof. Note that x C ∈ in (I(V M )) for C ∈ C(M) independent of the term order. Thus S (M) is a collection of sets τ ⊆ [n] such that C ⊆ τ for all C ∈ C(C). This means that τ is an independent set of M. It follows from the definition that V M * = (1, . . . , 1) − V M . The next lemma then yields Proposition 2.6. Denote by T i the reflection in the hyperplane {x : is a polynomial all whose terms strictly divide x α and thus are strictly smaller in any term order. It follows that in (I(V )) ⊆ in (I(T i (V ))) and thus S (V ) ⊆ S (T i (V )). However

Lex order, mapping cones, and bijections
Let be the lexicographic term order with x 1 ≻ x 2 ≻ · · · ≻ x n . That is x α ≺ x β if for the smallest i for which α i = β i we have α i > β i . The lexicographic order is strongly tied to elimination in ideals and projections of algebraic sets [CLO15, Ch. 3] and lends itself to inductive or recursive arguments.
Following lemma gives a recursive description of the standard monomials for a 0/1-point configuration. The recursive structure of standard monomials with respect to the lexicographic term order has been noted in various contexts; see, for example [Led09] for the description of the standard monomials for general point sets and the discussions in the last section of that paper. In the context of VC-dimensions, Anstee, Rónyai, and Sali call S lex (V ) order shattering and give a similar recursive description (and proof) in Theorem 4.3 of [ARS02].
Lemma 3.1. Let V ⊂ {0, 1} n be a non-empty 0/1-configuration and let ≺ be the lexicographic term order with x 1 ≻ x 2 ≻ · · · ≻ x n . Then Proof. Let us denote the right-hand side by S ′ . We first show that if τ ∈ S ′ , then there is a polynomial f τ ∈ I(V ) with in (f τ ) = x τ . Let τ ∈ S ′ be inclusion-minimal. If n ∈ τ , then τ ∈ S lex (V 0 )∪S lex (V 1 ) and there are polynomials f i τ ∈ I(V i ) with leading term x τ for i = 0, 1. By virtue of the lexicographic term order, the polynomial f τ = (1 − x n )f 0 τ + x n f 1 τ also has leading term x τ and vanishes on V .
If n ∈ τ , then σ : Consequently, the polynomial f τ := (x n − 1)f 0 σ has leading term x τ and vanishes on V . Let I ′ ⊆ I(V ) be the ideal generated by the polynomials f τ for τ ∈ S ′ . Then On the other hand, we have This shows that I ′ = I(V ) and proves the claim.
In this case, we can rephrase Lemma 3.1 as follows, which also yields (1.1).

Standard complexes for lattice path matroids
In this section, we discuss the proposed bijection in Corollary 1.2 for the max-minor class class of lattice path matroids. As we see below, this bijection closely relates to classical bijective combinatorics on lattice paths.
A lattice path C from (0, 0) to (d, n − d) in Z 2 is a sequence of d east steps e in direction (1, 0) and n − d north steps n in direction (0, 1). We denote the collection of all such path by L n,d . A path C ∈ L n,d may be represented as a word in {e, n}, and we refer to its i-th letter as C i . We refer to the actual line segment of a step in R 2 between its endpoints as its realization. For later reference, we also define a diagonal step d in direction (1, 1) and an empty step ε.
Say that U is weakly above L for U, L ∈ L n,d if U never goes below L. In terms of words this means that every prefix L contains at least as many east steps as the corresponding prefix of U. For two such paths, let L[U, L] ⊆ L n,d be the set of all lattice paths weakly between U and L, i.e, weakly below U and weakly above L. We refer to U as the upper boundary of L[U, L] and to L as the lower boundary.
If n and d are fixed, then we may identify L ∈ L n,d by its ordered collection of east steps We denote this by E(L) ≤ comp E(U).
It was observed by Bonin, de Mier, and Noy [BdMN03] that the collection of lattice path between U and L give rise to a matroid. The special case of Dyck paths was studied by Ardilla [Ard03]. Let U, L ∈ L n,d with U weakly above L. Then The goal of this section is to make this selection of east steps explicit by means of lattice path combinatorics. Concretely, this means to find a combinatorial statistic st associating to each path C ∈ L[U, L] a subset st(C) of its east steps E(C) such that Λ(E(C)) = st(C). 4.1. Simple version of main theorem. As a warm-up, we provide a description for the special case of the trivial lower boundary L tr = e · · · e d times n · · · n n−d times ∈ L n,d .
For a path C ∈ L n,d , the statistic st(C) is obtained by marking certain east steps as follows: scan through the word of C from left to right and mark the step C i = e if there are as many n's to the left of position i as there are unmarked e's. See Example 4.2 for an example of this marking process. As indicated in Figure 1, one may interpret the marked and unmarked east steps of a path C as well graphically by drawing the L tr -marking path of C. This path, mark L tr (C), of total length d consists of east steps e and diagonal steps d. It starts at (0, 0) and uses diagonal steps whenever possible without going above the path C, and otherwise uses east steps. The marked east steps of C are then those whose realizations are also east steps of the marking path mark L tr (C).  Note that st L tr (C) is constructed in a greedy-like manner and, more over, is independent of the upper boundary U.
Remark 4.3. As observed using the statistics database FindStat [RS + 19], one may as well define st L tr (C) using the following hook placements in the area between C and the lower boundary L tr . Scan the columns below a given path C ∈ L n,d from left to right and place, if possible, the corner box of a south-east hook into the north-most box below C in that column that is not already covered by other hooks. The columns of the placed corner boxes are then exactly the columns that contribute to the statistic. We refer to Figure 2 for an example of this procedure. In particular, the standard complex in this case can be described as

General version of main theorem.
After having given the simple version of the statistic for the trivial lower boundary L tr , we modify the definition of the marking path depending on the given lower boundary by introducing another intermediate path. We remark that it seems to be not possible to give a description in terms of hook placements as in Remark 4.3 for this general situation of non-trivial lower bounds.
Let C, L ∈ L n,d with C weakly above L. The L-demarcation path of C, dem L (C), is defined to start at (0, 0) with steps Graphically, this means that dem L (C) starts at (0, 0) and then is obtained by combining the e-coordinate of L with the n-coordinate of C. In particular, this implies that the L-demarcation path of C is weakly between L and C. Let C, L ∈ L n,d with C weakly above L. The L-marking path of C, mark L (C), consists of diagonal steps d, east steps e and north steps n. It goes from (0, 0) to (d, n − d) and uses diagonal steps whenever possible without going above the path C or below the L-demarcation path dem L (C). If it would go above C, it uses an east steps instead, and if it would go below dem L (C), it uses a north step instead. The marked east steps of C are then those whose realizations are also east steps of the marking path mark L (C).

8
Definition 4.6 (Combinatorial statistic, general version). Let C, L ∈ L n,d with C weakly above L and mark east steps of C using the L-marking path P = mark L (C). Then is the unmarked east steps in C.
Example 4.7. We mark east steps in the path C ∈ L 21,11 in Figure   In particular, the standard complex in this case can be described as Surprisingly, this combinatorial description yields the following property of the bijection Λ in the case of lattice path matroids.
Corollary 4.9. Let U, U ′ , L ∈ L n,d such that U is weakly above U ′ and U ′ is weakly above L. Then Proof. This is a direct consequence of Theorem 4.8.
In light of Proposition 2.6, this theorem also yields the following open problem. For a path C ∈ L n,d we write C * ∈ L n,n−d to be the path given by C * i = e if C i = n and vice versa.
Open problem 1. For U, L ∈ L n,d with U weakly above L, find a bijection The open problem is thus to provide a bijection without referring to the set J in the first place. By definition, the bijection in question must send L to U * and U to L * . In particular, it is not the obvious bijection C → C * . We also write L[U, L] e • and L[U, L] n • for the corresponding paths with the last east or, respectively, north step removed. If they exist, these truncated paths may clearly again be realized as lattice paths between boundaries: • , L e • ∈ L n−1,d−1 and U n • , L n • ∈ L n−1,d are obtained from U and L by respectively removing the last east or north step. (Note that in the first situation, the last east step of L might not be its last step and in the second, the last north step of U might not be its last step.) Proof of Theorem 4.4. We first prove that the simple version of the combinatorial statistic by showing the following theorem. Theorem 4.4 then follows with Theorem 1.1. We then have We prove this theorem in several steps and refer to Figure 4 for graphical illustrations of the given arguments.
Lemma 4.11. Let C ∈ L n,d with st L tr (C) = {j 1 < · · · < j ℓ }. Then Proof. We have st L tr (C) ⊆ E(C) and therefore ℓ = | st L tr (C)| ≤ |E(C)| = d. Moreover, the marking path mark L tr (C) consists of ℓ diagonal steps and of d − ℓ east steps and thus ends at the point (d, ℓ). Because C ends at (d, n − d) and is weakly above mark L tr (C), we also have ℓ ≤ n − d or, equivalently, d ≤ n − ℓ.
The second property j k ≥ 2k follows from the observation that, if there was a smallest k with j k < 2k, the realization of the j k -th step (which is an east step) in the path C would also be an east step of its marking path mark L tr (C) and thus j k / ∈ st L tr (C). Proof. Let C ∈ L n,d such that | st L tr (C)| = ℓ. As we have already seen in the proof of Lemma 4.11, the marking path of C ends at (d, ℓ). Because C ends at (d, n − d) and ℓ < n − d, the path C contains a unique last north step whose realization starts on the marking path. Replacing this north step by an east step yields a path C ′ ∈ L n,d+1 for which st L tr (C ′ ) = st L tr (C).
On the other hand, let C ′ ∈ L n,d+1 such that | st L tr (C ′ )| = ℓ. Because d + 1 > ℓ, the path C ′ contains a last east step whose realization is also an east step of its marking path mark L tr (C ′ ). Replacing this east step by a north step yields a path C ∈ L n,d with st L tr (C) = st L tr (C ′ ).
Those two operations are inverses of each other and are thus the desired bijection. This can be seen as follows: Consider C and its marking path P = mark L tr (C), and also C ′ and its marking path P ′ = mark L tr (C ′ ), and let i be the index of the north step in C that is replaced in C ′ by an east step (or vice versa). Then the relative positions of C, P and, respectively, the relative positions of C ′ , P ′ coincide, except that C ′ is horizontally one step closer to P ′ than C is to P . We refer to Figure 4 for several examples.  Corollary 4.14 now implies Theorem 4.10.
Proof of Theorem 4.8. We next generalize and modify the arguments for the simple version to obtain the general version. We show the following theorem and Theorem 4.8 then follows with Theorem 1.1. We first adapt the definition of the demarcation path and the marking path to the slightly more general setting of C ∈ L n,d and L ∈ L n,d ′ with d ≤ d ′ . The definition of C being weakly above L generalizes verbatim and d ≤ d ′ is a necessary condition in this case. One may then extend C and L by appending d ′ − d many east steps to C and d ′ − d many north steps to L to obtain two paths in L n+d ′ −d,d ′ . The definitions of the L-demarcation path and the L-marking path of C are given by using these extensions. Figure 5 shows one such example.
Unfortunately, we do not have an analogue of Lemma 4.11 in the general situation. We nonetheless still have the following lemma.
Lemma 4.16. Fix n, d < d ′ and J ⊆ {1, . . . , n} with |J| = ℓ ≤ d and L ∈ L n,d ′ . Then there is an explicit bijection    C ∈ L n,d : In the lemma, we mean by (d, n − d) / ∈ mark L (C) that the marking path mark L (C) of C does not go through the endpoint (d, n − d) of C.
Proof. The proof of Lemma 4.12 generalizes verbatim: Let C ∈ L n,d be weakly above L such that | st L (C)| = ℓ. Because (d, n − d) / ∈ mark L (C), the path C contains a unique last north step whose realization start on the marking path mark L (C). Replacing this north step by an east step yields a path C ′ ∈ L n,d+1 with st L (C ′ ) = st L (C) which is also weakly above L.
On the other hand, let C ′ ∈ L n,d+1 be weakly above L such that | st L (C ′ )| = ℓ. Because d + 1 > ℓ, the path C ′ contains a last east step whose realization is also an east step of its marking path mark L (C ′ ). Replacing this east step by a north step yields a path C ∈ L n,d with st L (C) = st L (C ′ ) and (d, n − d) / ∈ mark L (C).
Those two operations are inverses of each other and are thus the desired bijection. This can be seen as follows: Consider C, D = dem L (C) and M = mark L (C), and also C ′ , D ′ = dem L (C ′ ) and M ′ = mark L (C ′ ), and let i be the index of the north step in C that is replaced in C ′ by an east step (or vice versa). Then the relative positions of C, D, M and, respectively, the relative positions of C ′ , D ′ , M ′ coincide, except that C ′ is horizontally one step closer to D ′ and to M ′ than C is to D and to M. We refer to Figure 5 for two examples.
Proposition 4.17. Fix n, d < d ′ and J ⊆ {1, . . . , n}. Let L ∈ L n,d ′ such that the unique path E ∈ L n,ℓ with E(C) = J is weakly above L. Set k to be the smallest nonnegative integer such that the point (ℓ + k, n − ℓ) is contained in mark L (E). Then there exists C ∈ L n,d with st L (C) = J if and only if the two conditions st L (E) = J and ℓ ≤ d ≤ ℓ + k.
are both satisfied. Moreover, the path with this property is unique in this case.
Proof. We have seen in Lemma 4.16 that these two conditions are both necessary and sufficient for such a path to exist. The uniqueness is also a direct consequence of the previous lemma because the path E is obviously the unique path in L n,ℓ with st L (E) = J = E(E).
One may easily check that Proposition 4.17 indeed reduces to Proposition 4.13 for the trivial lower bound. The parameter k introduced in the previous proposition is the