arXiv subjects

Ross Geoghegan

Publications and source records attributed to Ross Geoghegan.

18 recordsLinked to original sources

arXiv2024

Topological posets and tropical phased matroids

For a discrete poset $\mathcal X$ McCord proved that the natural map $|{\mathcal X}|\to {\mathcal X}$ from the order complex to the poset equipped with the Up topology is a weak homotopy equivalence. Much later, Zivaljević defined the notion of order complex for a topological poset. For a large class of such topological posets we prove the analog of McCord's theorem, namely that the natural map from the order complex to the topological poset with the Up topology is a weak homotopy equivalence. A familiar topological example is the Grassmann poset of proper non-zero linear subspaces of R^{n+1} partially ordered by inclusion. But our motivation in topological combinatorics is to apply the theorem to posets associated with tropical phased matroids over the tropical phase hyperfield, and in particular to elucidate the tropical version of the MacPhersonian Conjecture. This is explained in Section 2.

math.CO↗

arXiv2021

The Up Topology for Mirrored Topological Posets

For a discrete poset $\mathcal X$, McCord proved that the natural map $|{\mathcal X}|\to {\mathcal X}$, from the order complex to the poset with the Up topology, is a weak homotopy equivalence. Much later, uZivaljević defined the notion of order complex for a topological poset. For a large class of such topological posets we prove the analog of McCord's theorem, namely that the natural map from the order complex to the topological poset with the Up topology is a weak homotopy equivalence. An example is the Grassmann poset of proper non-zero linear subspaces of $\R^{n+1}$. Here, Vassiliev had computed the homotopy type of the order complex. Our theorem allows us to transfer that information (up to weak homotopy type) to the Grassmann poset itself with the Up topology.

math.CO↗

arXiv2020

On semistability of $CAT(0)$ groups

Does every one-ended $CAT(0)$ group have semistable fundamental group at infinity? As we write, this is an open question. Let $G$ be such a group acting geometrically on the proper $CAT(0)$ space $X$. In this paper we show that in order to establish a positive answer to the question it is only necessary to check that any two geodesic rays in $X$ are properly homotopic. We then show that if the answer to the question is negative, with $(G,X)$ a counter-example, then the boundary of $X$, $\del X$ with the cone topology, must have a weak cut point. This is of interest because a theorem of Papasoglu and the second-named author \cite{PS} has established that there cannot be an example of $(G,X)$ where $\del X$ has a cut point. Thus, the search for a negative answer comes down to the difference between cut points and weak cut points. We also show that the Tits ball of radius $\fracπ{2}$ about that weak cut point is a "cut set" in the sense that it separates $\del X$. Finally, we observe that if a negative example $(G, X)$ exists then $G$ is rank 1.

math.GR↗

arXiv2018

Topological properties of spaces admitting a coaxial homeomorphism

Wright showed that, if a 1-ended simply connected locally compact ANR Y with pro-monomorphic fundamental group at infinity admits a proper Z-action, then that fundamental group at infinity can be represented by an inverse sequence of finitely generated free groups. Geoghegan and Guilbault strengthened that result, proving that Y also satisfies the crucial "semistability" condition. Here we get a stronger theorem with weaker hypotheses. We drop the pro-monomorphic hypothesis and simply assume that the Z-action is generated by what we call a "coaxial" homeomorphism. In the pro-monomorphic case every proper Z-action is generated by a coaxial homeomorphism, but coaxials occur in far greater generality (often embedded in a cocompact action). When the generator is coaxial, we obtain the sharp conclusion: Y is proper 2-equivalent to the product of a locally finite tree with a line. Even in the pro-monomorphic case this is new: it says that, from the viewpoint of fundamental group at infinity, the end of Y looks like the suspension of a totally disconnected compact set.

math.GT↗

arXiv2018

Higher horospherical limit sets for G-modules over CAT(0) spaces

The Sigma-invariants of Bieri-Neumann-Strebel and Bieri-Renz involve an action of a discrete group G on a geometrically suitable space M. In the early versions, M was always a finite-dimensional Euclidean space on which G acted by translations. A substantial literature exists on this, connecting the invariants to group theory and to tropical geometry (which, actually, Sigma-theory anticipated). More recently, we have generalized these invariants to the case where M is a proper CAT(0) space on which G acts by isometries. The "0th stage" of this was developed in our paper [BG16]. The present paper provides a higher-dimensional extension of the theory to the "nth stage" for any n.

math.GR↗

arXiv2017

On Thompson's group T and algebraic K-theory

Using a theorem of Lück-Reich-Rognes-Varisco, we show that the Whitehead group of Thompson's group T is infinitely generated, even when tensored with the rationals. To this end we describe the structure of the centralizers and normalizers of the finite cyclic subgroups of T, via a direct geometric approach based on rotation numbers. This also leads to an explicit computation of the source of the Farrell-Jones assembly map for the rationalized higher algebraic K-theory of the integral group ring of T.

math.GT↗

arXiv2017

Non-cocompact Group Actions and $π_1$-Semistability at Infinity

A finitely presented 1-ended group $G$ has {\it semistable fundamental group at infinity} if $G$ acts geometrically on a simply connected and locally compact ANR $Y$ having the property that any two proper rays in $Y$ are properly homotopic. This property of $Y$ captures a notion of connectivity at infinity stronger than "1-ended", and is in fact a feature of $G$, being independent of choices. It is a fundamental property in the homotopical study of finitely presented groups. While many important classes of groups have been shown to have semistable fundamental group at infinity, the question of whether every $G$ has this property has been a recognized open question for nearly forty years. In this paper we attack the problem by considering a proper {\it but non-cocompact} action of a group $J$ on such an $Y$. This $J$ would typically be a subgroup of infinite index in the geometrically acting over-group $G$; for example $J$ might be infinite cyclic or some other subgroup whose semistability properties are known. We divide the semistability property of $G$ into a $J$-part and a "perpendicular to $J$" part, and we analyze how these two parts fit together. Among other things, this analysis leads to a proof (in a companion paper) that a class of groups previously considered to be likely counter examples do in fact have the semistability property.

math.GR↗

arXiv2017

Semistability and CAT(0) Geometry

We explain why semistability of a one-ended proper CAT(0) space can be determined by the geodesic rays. This is applied to boundaries of CAT(0) groups.

math.GR↗

arXiv2016

Limit sets for modules over groups on CAT(0) spaces -- from the Euclidean to the hyperbolic

The observation that the 0-dimensional Geometric Invariant $Σ^{0}(G;A)$ of Bieri-Neumann-Strebel-Renz can be interpreted as a horospherical limit set opens a direct trail from Poincaré's limit set $Λ(Γ)$ of a discrete group $Γ$ of Möbius transformations (which contains the horospherical limit set of $Γ$) to the roots of tropical geometry (closely related to $Σ^{0}(G;A)$ when G is abelian). We explore this trail by introducing the horospherical limit set, $Σ(M;A)$, of a G-module A when G acts by isometries on a proper CAT(0) metric space M. This is a subset of the boundary at infinity of M. On the way we meet instances where $Σ(M;A)$ is the set of all conical limit points, the complement of a spherical building, the complement of the radial projection of a tropical variety, or (via the Bieri-Neumann-Strebel invariant) where it is closely related to the Thurston norm.

math.GR↗

arXiv2011

Topological properties of spaces admitting free group actions

In 1992, David Wright proved a remarkable theorem about which contractible open manifolds are covering spaces. He showed that if a one-ended open manifold M has pro-monomorphic fundamental group at infinity which is not pro-trivial and is not stably Z, then M does not cover any manifold (except itself). In the non-manifold case, Wright's method showed that when a one-ended, simply connected, locally compact ANR X with pro-monomorphic fundamental group at infinity admits an action of Z by covering transformations then the fundamental group at infinity of X is (up to pro-isomorphism) an inverse sequence of finitely generated free groups. We improve upon this latter result, by showing that X must have a stable finitely generated free fundamental group at infinity. Simple examples show that a free group of any finite rank is possible. We also prove that if X (as above), admits a non-cocompact action of Z+Z by covering transformations, then X is simply connected at infinity. Corollary: Every finitely presented one-ended group G which contains an element of infinite order satisfies exactly one of the following: 1) G is simply connected at infinity; 2) G is virtually a surface group; 3) The fundamental group at infinity of G is not pro-monomorphic. Our methods also provide a quick new proof of Wright's open manifold theorem.

math.GT↗

arXiv2009

Sigma Invariants of Direct Products of Groups

The Product Conjecture for the homological Bieri-Neumann-Strebel-Renz invariants is proved over a field. Under certain hypotheses the Product Conjecture is shown to also hold over Z, even though D. Schuetz has recently shown that the Conjecture is false in general over Z. Our version over Z is applied in a joint paper with D. Kochloukova to derive new information about subgroups of Thompson's group F, namely that F has subgroups F_m which are not of type F_{m+1}.

math.GR↗

arXiv2008

Closed 1-Forms in Topology and Geometric Group Theory

In this article we describe relations of the topology of closed 1-forms to the group theoretic invariants of Bieri-Neumann-Strebel-Renz. Starting with a survey, we extend these Sigma invariants to finite CW- complexes and show that many properties of the group theoretic version have analogous statements. In particular we show the relation between Sigma invariants and finiteness properties of certain infinite covering spaces. We also discuss applications of these invariants to the Lusternik- Schnirelmann category of a closed 1-form and to the existence of a non- singular closed 1-form in a given cohomology class on a high-dimensional closed manifold.

math.AT↗

arXiv2008

The Sigma Invariants of Thompson's Group F

Thompson's group F is the group of all increasing dyadic piecewise linear homeomorphisms of the closed unit interval. We compute Sigma^m(F) and Sigma^m(F;Z), the homotopical and homological Bieri-Neumann-Strebel-Renz invariants of F, and we show that Sigma^m(F) = Sigma^m(F;Z). As an application, we show that, for every m, F has subgroups of type F_{m-1} which are not of type F_{m}.

math.GR↗

arXiv2005

Associativity and Thompson's Group

Given a set S equipped with a binary operation (we call this a "bracket algebra") one may ask to what extent the binary operation satisfies some of the consequences of the associative law even when it is not actually associative? We define a subgroup Assoc(S) of Thompson's Group F for each bracket algebra S, and we interpret the size of Assoc(S) as determining the amount of associativity in S - the larger Assoc(S) is, the more associativity holds in S. When S is actually associative, Assoc(S) = F; that is the trivial case. In general, it turns out that only certain subgroups of F can occur as Assoc(S) for some S, and we describe those subgroups precisely. We then explain what happens in some familiar examples: Lie algebras with the Lie bracket as binary operation, groups with the commutator bracket as binary operation, the Cayley numbers with their usual multiplication, as well as some less familiar examples. In the case of a group G, with the commutator bracket as binary operation, it is better to think of the "virtual size of G", determined by all the groups Assoc(H) such that H is a subgroup of finite index in G. This gives a way of partitioning groups into "small", "intermediate" and "large" - a partition suggestive of, but different from, traditional measures of a group's size such as growth, isoperimetric inequality and "amenable vs. non-amenable"

math.GR↗

arXiv2004

Cocompact Proper CAT(0) Spaces

This paper is about geometric and topological properties of a proper CAT(0) space $X$ which is cocompact - i.e. which has a compact generating domain with respect to the full isometry group. It is shown that geodesic segments in $X$ can "almost" be extended to geodesic rays. A basic ingredient of the proof of this geometric statement is the topological theorem that there is a top dimension $d$ in which the compactly supported integral cohomology of $X$ is non-zero. It is also proved that the boundary-at-infinity of $X$ (with the cone topology) has Lebesgue covering dimension $d-1$. It is not assumed that there is any cocompact discrete subgroup of the isometry group of $X$; however, a corollary for that case is that "the dimension of the boundary" is a quasi- isometry invariant of CAT(0) groups. (By contrast, it is known that the topological type of the boundary is not unique for a CAT(0) group.)

math.MG↗

arXiv1998

Connectivity properties of group actions on non-positively curved spaces I: Controlled connectivity and openness results

Let G be a group and let M be a CAT(0) proper metric space (e.g. a simply connected complete Riemannian manifold of non-positive sectional curvature or a locally finite tree). Isometric actions of G on M are (by definition) points in the space R := Hom(G, Isom(M)) with the compact open topology. Sample theorems: 1. The cocompact actions form an open subset of R. 2. The cocompact actions with discrete orbits whose point-stabilizers have type F_n form an open subset of the subspace of R consisting of all actions with discrete orbits. (F_1 means finitely generated, F_2 means finitely presented etc.) The key idea is to introduce a new "controlled topology" invariant of such actions - dependent on n - which is unfamiliar when the orbits are not discrete but which becomes familiar (cf 2.) when the orbits are discrete. (This is the first of two papers.)

math.GR↗

arXiv1998

Connectivity properties of group actions on non-positively curved spaces II: The geometric invariants

This is the second of two papers but has been written so as to have minimal dependence on the first paper (which is also on this archive). Let G be a group and let M be a CAT(0) proper metric space (e.g. a simply connected complete Riemannian manifold of non-positive sectional curvature or a locally finite tree). Assume G is of type F_n (type F_1 is finitely generated, type F_2 is finitely presented etc.) The "boundary", bdM, of M at infinity has two customary topologies - the compact "cone" topology and the Tits metric topology. We associate with any isometric action of G on M two subsets of bdM, both dependent on n. These subsets consist of those points of bdM at which - in two senses - the action is "controlled (n-1)-connected". One of these sets is open in the Tits metric topology. Even in classical cases like familiar groups of isometries of the hyperbolic plane or of a locally finite tree these sets seem to be new and interesting invariants. The "SIGMA-theory" of Bieri-Neumann-Strebel-Renz is recovered in the special case in which M is G(abelianized) tensor R with the translation action.

math.GR↗

arXiv1998

A Hochschild homology Euler characteristic for circle actions

We define a "circle Euler characteristic" of a circle action on a compact manifold or finite complex X. It lies in the first Hochschild homology group of ZG where G is the fundamental group of X. It is analogous in many ways to the ordinary Euler characteristic. One application is an intuitively satisfying formula for the Euler class (integer coefficients) of the normal bundle to a smooth circle action without fixed points on a manifold. In the special case of a 3-dimensional Seifert fibered space, this formula is particularly effective. \~

math.KT↗