arXiv subjects

Volkmar Welker

Publications and source records attributed to Volkmar Welker.

At least 19 recordsLinked to original sources

arXiv2026

Pairs of lattice complements with complementary homology

In this paper we study whether there is a global to local principle for the homology of order complexes of finite lattices. We explore whether the non-vanishing of the homology of a lattice in degree $a+b-2$ forces the existence of a pair of lattice-complements whose open intervals below have non-vanishing homology in degrees $a$ and $b$ respectively. We consider the same question also in the language of multigraded free resolutions. We provide positive evidence for the existence of such pairs and give counterexamples to strengthenings of the question.

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arXiv2026

Simplicial Complexes of Antichains in Root Posets and Related Combinatorics of Dyck Paths

For a crystallographic root system ${\mathfrak D}$ we consider the simplicial complex $Δ_{\mathfrak D}$ of all antichains in the root poset of ${\mathfrak D}$. We show that $Δ_{\mathfrak D}$ is shellable if and only if ${\mathfrak D}$ is $A_n$, $B_n$, $D_3$ or $G_2$. Since antichains in types $A_n$ and $B_n$ can be identified with Dyck paths and symmetric Dyck paths, respectively, this yields a simplicial complex on Dyck paths. Indeed, in type $A_n$, shellability can be extended to rational Dyck paths. The $f$- and $h$-triangles then yield statistics on (symmetric/rational) Dyck paths. We determine these statistics for $A_n$ and $B_n$ and leave the case of rational Dyck paths as an open problem.

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arXiv2026

Subadditivity of shifts, Eilenberg-Zilber shuffle products and homology of lattices

We show that the maximal shifts in the minimal free resolution of the quotients of a polynomial ring by a monomial ideal are subadditive as a function of the homological degree. This answers a question that has received some attention in recent years. To do so, we define and study a new model for the homology of posets, given by the so called synor complex. We also introduce an Eilenberg-Zilber type shuffle product on the simplicial chain complex of lattices. Combining these concepts we prove that the existence of a nonzero homology class for a lattice forces certain nonzero homology classes in lower intervals. This result then translates into properties of the minimal free resolution. In particular, it yields a strengthening of the original subadditivity statement.

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arXiv2026

Homological algebra and poset versions of the Garland method

Garland introduced a vanishing criterion for a characteristic zero cohomology group of a locally finite and locally connected simplicial complex. The criterion is based on the spectral gaps of the graph Laplacians of the links of faces and has turned out to be effective in a wide range of examples. In this note we extend the approach to include a range of non-simplicial (co)chain complexes associated to combinatorial structures we call Garland posets and elaborate further on the case of cubical complexes.

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arXiv2026

Generalized binomial edge ideals are Cartwright-Sturmfels

Binomial edge ideals associated to a simple graph G were introduced by Herzog and collaborators and, independently, by Ohtani. They became an ``instant classic" in combinatorial commutative algebra with more than 100 papers devoted to their investigation over the past 15 years. They exhibit many striking properties, including being radical and, moreover, Cartwright-Sturmfels. Using the fact that binomial edge ideals can be seen as ideals of 2-minors of a matrix of variables with two rows, generalized binomial edge ideals of 2-minors of matrices of m rows were introduced by Rauh and proved to be radical. The goal of this paper is to prove that generalized binomial edge ideals are Cartwright-Sturmfels. On the way we provide results on ideal constructions preserving the Cartwright-Sturmfels property. We also give examples and counterexamples to the Cartwright-Sturmfels property for higher minors.

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arXiv2025

Posets of decompositions in spherical buildings

We propose definitions of the common bases complex, the poset of decompositions, and the poset of partial decompositions for arbitrary spherical buildings. We show that the poset of decompositions is Cohen-Macaulay, and that the poset of partial decompositions is spherical and homotopy equivalent to the common bases complex. To prove these results, we rely on the concepts of opposition, Levi spheres, and convexity in buildings. In particular, our results extend the already known constructions for the linear case (vector spaces) to arbitrary buildings. As a byproduct, we see that the poset of ordered partial decompositions carries the square of the Steinberg representation.

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arXiv2025

On the homology of simplicial and cubical sets with symmetries

We study the homology of simplicial and cubical sets with symmetries. These are simplicial and cubical sets with additional maps expressing the symmetries of simplices and cubes. We consider the chain complex computing the homology groups in either case. We show for coefficients in fields of characteristic $0$ that the sub-complex generated by degeneracies (simplicial case) or connections (cubical case) together with all $x - sgn(t)tx$ for symmetries $t$ and chains $x$ is acyclic. In particular, it follows that quotienting by this sub-complex yields a chain complex with isomorphic homology. The latter leads to structural insight and a speedup in explicit computations. We also exhibit examples which show that acyclicity does not hold for general coefficient rings $R$.

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arXiv2025

Notes on the topology of independence structures

Following Welsh, a pre-independence space (pi-space) is a set $M$ together with a non-empty collection $I(M)$ of subsets of $M$, called independent sets, which is closed under taking subsets, and finite independent sets satisfy the exchange property from matroid theory. We show that $I(M)$, viewed as a poset, is contractible if it is infinite-dimensional, and Cohen-Macaulay otherwise. Moreover, the proper part of the associated poset of flats is also contractible in the infinite-dimensional case, and Cohen-Macaulay otherwise. These results generalize those for independence complexes and geometric lattices of (finite) matroids.

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arXiv2025

Posets arising from decompositions of objects in a monoidal category

Given a symmetric monoidal category $C$ with product $\sqcup$, where the neutral element for the product is an initial object, we consider the poset of $\sqcup$-complemented subobjects of a given object $X$. When this poset has finite height, we define decompositions and partial decompositions of $X$ which are coherent with $\sqcup$, and order them by refinement. From these posets, we define complexes of frames and partial bases, augmented Bergman complexes and related ordered versions. We propose a unified approach to the study of their combinatorics and homotopy type, establishing various properties and relations between them. Via explicit homotopy formulas, we will be able to transfer structural properties, such as Cohen-Macaulayness. In well-studied scenarios, the poset of $\sqcup$-complemented subobjects specializes to the poset of free factors of a free group, the subspace poset of a vector space, the poset of non-degenerate subspaces of a vector space with a non-degenerate form, and the lattice of flats of a matroid. The decomposition and partial decomposition posets, the complex of frames and partial bases together with the ordered versions, either coincide with well-known structures, generalize them, or yield new interesting objects. In these particular cases, we provide new results along with open questions and conjectures.

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arXiv2025

On a question about real rooted polynomials and f-polynomials of simplicial complexes

For a polynomial $f(t) = 1+f_0t+\cdots +f_{d-1}t^d$ with positive integer coefficients Bell and Skandera ask if real rootedness of f(t) implies that there is a simplicial complex with f-vector $(1,f_0 \ldots,f_{d-1})$. In this paper we discover properties implied by the real rootedness of f(t) in terms of the binomial representation $f_i = \binom{x_{i+1}}{i+1}, i \geq 0$. We use these to provide a sufficient criterion for a positive answer to the question by Bell and Skandera. We also describe two further approaches to the conjecture and use one to verify that some well studied real rooted classical polynomials are f-polynomials. Finally, we provide a series of results showing that the set of f-vectors of simplicial complexes is closed under constructions also preserving real rootedness of their generating polynomials.

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arXiv2024

Total positivity and two inequalities by Athanasiadis and Tzanaki

Let $Δ$ be a $(d-1)$-dimensional simplicial complex and $h^ Δ= (h_0^ Δ,\ldots, h_d^ Δ)$ its $h$-vector. For a face uniform subdivision operation ${\mathcal F}$ we write $Δ_{\mathcal F}$ for the subdivided complex and $H_{\mathcal F}$ for the matrix such that $h^ {Δ_{\mathcal F}} = H_{\mathcal F} h^ Δ$. In connection with the real rootedness of symmetric decompositions Athanasiadis and Tzanaki studied for strictly positive $h$-vectors the inequalities $\frac{h_0^ Δ}{h_1^ Δ} \leq \frac{h_1^Δ}{h_{d-1}^ Δ} \leq \cdots \leq \frac{h_d^ Δ}{h_0^Δ}$ and $\frac{h_1^Δ}{h_{d-1}^Δ} \geq \cdots \geq \frac{h_{d-2}^Δ}{h_2^Δ} \geq \frac{h_{d-1}^Δ}{h_1^Δ}$. In this paper we show that if the inequalities holds for a simplicial complex $Δ$ and $H_{\mathcal F}$ is TP$_2$ (all entries and two minors are non-negative) then the inequalities hold for $Δ_{\mathcal F}$. We prove that if ${\mathcal F}$ is the barycentric subdivision then $H_{\mathcal F}$ is TP$_2$. If ${\mathcal F}$ is the $r$\textsuperscript{th}-edgewise subdivision then work of Diaconis and Fulman shows $H_{\mathcal F}$ is TP$_2$. Indeed in this case by work of Mao and Wang $H_{\mathcal F}$ is even TP.

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arXiv2024

The common basis complex and the partial decomposition poset

For a finite-dimensional vector space $V$, the common basis complex of $V$ is the simplicial complex whose vertices are the proper non-zero subspaces of $V$, and $σ$ is a simplex if and only if there exists a basis $B$ of $V$ that contains a basis of $S$ for all $S\in σ$. This complex was introduced by Rognes in 1992 in connection with stable buildings. In this article, we prove that the common basis complex is homotopy equivalent to the proper part of the poset of partial direct sum decompositions of $V$. Moreover, we establish this result in a more general combinatorial context, including the case of free groups, matroids, vector spaces with non-degenerate sesquilinear forms, and free modules over commutative Hermite rings, such as local rings or Dedekind domains.

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arXiv2023

Real polynomials with constrained real divisors. I. Fundamental groups

In the late 80s, V.~Arnold and V.~Vassiliev initiated the topological study of the space of real univariate polynomials of a given degree d and with no real roots of multiplicity exceeding a given positive integer. Expanding their studies, we consider the spaces of real monic univariate polynomials of degree d whose real divisors avoid sequences of root multiplicities taken from a given poset of compositions which is closed under certain natural combinatorial operations. In this paper, we concentrate on the fundamental group of such spaces. We find explicit presentations for the fundamental groups in terms of generators and relations and show that in a number of cases they are free with rank bounded from above by a quadratic function in d. We also show that the fundamental group stabilizes for d large. We further show that the fundamental groups admit an interpretation as special bordisms of immersions of 1-manifolds into the cylinder S^1 \times R, whose images avoid the tangency patterns from the poset with respect to the generators of the cylinder.

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arXiv2023

Homotopy properties of the complex of frames of a unitary space

Let $V$ be a finite dimensional vector space equipped with a non-degenerate Hermitian form over a field $\mathbb{K}$. Let $\mathcal{G}(V)$ be the graph with vertex set the $1$-dimensional non-degenerate subspaces of $V$ and adjacency relation given by orthogonality. We give a complete description of when $\mathcal{G}(V)$ is connected in terms of the dimension of $V$ and the size of the ground field $\mathbb{K}$. Furthermore, we prove that if $\dim(V) > 4$ then the clique complex $\mathcal{F}(V)$ of $\mathcal{G}(V)$ is simply connected. For finite fields $\mathbb{K}$, we also compute the eigenvalues of the adjacency matrix of $\mathcal{G}(V)$. Then by Garland's method, we conclude that $\tilde{H}_m(\mathcal{F}(V);\mathbb{k}) = 0$ for all $0\leq m\leq \dim(V)-3$, where $\mathbb{k}$ is a field of characteristic $0$, provided that $\dim(V)^2 \leq |\mathbb{K}|$. Under these assumptions, we deduce that the barycentric subdivision of $\mathcal{F}(V)$ deformation retracts to the order complex of the certain rank selection of $\mathcal{F}(V)$ which is Cohen-Macaulay over $\mathbb{k}$. Finally, we apply our results to the Quillen poset of elementary abelian $p$-subgroups of a finite group and to the study of geometric properties of the poset of non-degenerate subspaces of $V$ and the poset of orthogonal decompositions of $V$.

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arXiv2022

Hypergraph LSS-ideals and coordinate sections of symmetric tensors

Let K be a field, [n]= {1,...,n} and H=([n],E) be a hypergraph. For an integer d >= 1 the Lovasz-Saks-Schrijver ideal (LSS-ideal) L_H^K (d) in K[y_{ij}~:~(i,j) \in [n] x [d]] is the ideal generated by the polynomials $f^{(d)}_{e}= \sum\limits_{j=1}^{d} \prod\limits_{i \in e} y_{ij}$ for edges e of H. In this paper for an algebraically closed field K and a k-uniform hypergraph H=([n],E) we employ a connection between LSS-ideals and coordinate sections of the closure of the set S_{n,k}^d of homogeneous degree k symmetric tensors in n variables of rank <= d to derive results on the irreducibility of its coordinate sections. To this end we provide results on primality and the complete intersection property of L_H^K (d). We then use the combinatorial concept of positive matching decomposition of a hypergraph H to provide bounds on when L_H^K(d) turns prime to provide results on the irreducibility of coordinate sections of S_{n, k}^d.

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arXiv2022

Powers of monomial ideals with characteristic-dependent Betti numbers

We explore the dependence of the Betti numbers of monomial ideals on the characteristic of the field. A first observation is that for a fixed prime $p$ either the $i$-th Betti number of all high enough powers of a monomial ideal differs in characteristic $0$ and in characteristic $p$ or it is the same for all high enough powers. In our main results we provide constructions and explicit examples of monomial ideals all of whose powers have some characteristic-dependent Betti numbers or whose asymptotic regularity depends on the field. We prove that, adding a monomial on new variables to a monomial ideal, allows to spread the characteristic dependence to all powers. For any given prime $p$, this produces an edge ideal such that the Betti numbers of all its powers over $\mathbb{Q}$ and over $\mathbb{Z}_p$ are different. Moreover, we show that, for every $r \geq 0$ and $i \geq 3$ there is a monomial ideal $I$ such that some coefficient in a degree $\geq r$ of the Kodiyalam polynomials $\mathfrak P_3(I),\ldots,\mathfrak P_{i+r}(I)$ depends on the characteristic. We also provide a summary of related results and speculate about the behaviour of other combinatorially defined ideals.

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arXiv2021

Spaces of polynomials with constrained real divisors, II. (Co)homology & stabilization

In the late 80s, V.~Arnold and V.~Vassiliev initiated the topological study of the space of real univariate polynomials of a given degree which have no real roots of multiplicity exceeding a given positive integer. Expanding their studies, we consider the spaces P^{cΘ}_d of real monic univariate polynomials of degree d whose real divisors avoid given sequences of root multiplicities. These forbidden sequences are taken from an arbitrary poset Θof compositions that are closed under certain natural combinatorial operations. We reduce the computation of the homology H_*(P^{cΘ}_d) to the computation of the homology of a differential complex, defined purely combinatorially in terms of the given closed poset Θ. We also obtain the stabilization results about H^\ast(P^{c Θ}_d), as d goes to infinity. These results are deduced from our description of the homology of spaces B^{c Θ}_d whose points are binary real homogeneous forms, considered up to projective equivalence, with similarly Θ-constrained real divisors. In particular, we exhibit differential complexes that calculate the homology of these spaces and obtain some stabilization results for H^*(B^{c Θ}_d), as d goes to infinity. In particular, we compute the homology of the discriminants of projectivized binary real forms for which there is at least one line on which the form vanishes with multiplicity >= 2 and of their complements in \cB_d \cong RP^d.

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arXiv2021

On the homeomorphism and homotopy type of complexes of multichains

In this paper we define and study for a finite partially ordered set P a class of simplicial complexes on the set P_r of r-element multichains from P. The simplicial complexes depend on a strictly monotone function from [r] to [2r]. We show that there exactly 2^r such functions which yield subdivisions of the order complex of P of which 2^{r-1} are pairwise different. Within this class are for example the order complexes of the interval and the zig-zag poset of P and the rth edgewise subdivision of the order complex of P. We also exhibit a large subclass for which our simplicial complexes are order complexes and homotopy equivalent to the order complex of P.

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