Borel Serre Compactification Research Articles

The reductive Borel–Serre compactification as a model for unstable algebraic K-theory

Let A be an associative ring and M a finitely generated projective A-module. We introduce a category RBS(M)\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$${\ ext {RBS}}(M)$$\\end{document} and prove several theorems which show that its geometric realisation functions as a well-behaved unstable algebraic K-theory space. These categories RBS(M)\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$${\ ext {RBS}}(M)$$\\end{document} naturally arise as generalisations of the exit path ∞\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\infty $$\\end{document}-category of the reductive Borel–Serre compactification of a locally symmetric space, and one of our main techniques is to find purely categorical analogues of some familiar structures in these compactifications.

The Stratified Homotopy Type of the Reductive Borel–Serre Compactification

Abstract We identify the exit path $\infty $-category of the reductive Borel–Serre compactification as the nerve of a $1$-category defined purely in terms of rational parabolic subgroups and their unipotent radicals. As immediate consequences, we identify the fundamental group of the reductive Borel–Serre compactification, recovering a result of Ji–Murty–Saper–Scherk, and we obtain a combinatorial incarnation of constructible complexes of sheaves on the reductive Borel–Serre compactification as elements in a derived functor category.

Bianchi’s additional symmetries

In a 2012 note in Comptes Rendus Mathématique, the author did try to answer a question of Jean-Pierre Serre; it has recently been announced that the scope of that answer needs an adjustment, and the details of this adjustment are given in the present paper. The original question is the following. Consider the ring of integers \(\mathcal {O}\) in an imaginary quadratic number field, and the Borel–Serre compactification of the quotient of hyperbolic 3–space by \(\mathrm {SL_2}(\mathcal {O})\). Consider the map \(\alpha \) induced on homology when attaching the boundary into the Borel–Serre compactification. How can one determine the kernel of \(\alpha \) (in degree 1) ? Serre used a global topological argument and obtained the rank of the kernel of \(\alpha \). He added the question what submodule precisely this kernel is.

Feynman amplitudes on moduli spaces of graphs

This article introduces moduli spaces of coloured graphs on which Feynman amplitudes can be viewed as “discrete” volume densities. The basic idea behind this construction is that these moduli spaces decompose into disjoint unions of open cells on which parametric Feynman integrals are defined in a naturalway. Renormalisation of an amplitude translates then into the task of assigning to every cell a finite volume such that boundary relations between neighboring cells are respected. It is shown that this can be organized systematically using a type of Borel–Serre compactification of these moduli spaces. The key point is that in each compactified cell the newly added boundary components have a combinatorial description that resembles the forest structure of subdivergences of the corresponding Feynman diagram.

Analytic torsion and Reidemeister torsion of hyperbolic manifolds with cusps

On an odd-dimensional oriented hyperbolic manifold of finite volume with strongly acyclic coefficient systems, we derive a formula relating analytic torsion with the Reidemeister torsion of the Borel–Serre compactification of the manifold. In a companion paper, this formula is used to derive exponential growth of torsion in cohomology of arithmetic groups.

Hecke operators in $KK$-theory and the $K$-homology of Bianchi groups

Let \Gamma be a torsion-free arithmetic group acting on its associated global symmetric space X . Assume that X is of non-compact type and let \Gamma act on the geodesic boundary \partial X of X . Via general constructions in KK -theory, we endow the K -groups of the arithmetic manifold X / \Gamma , of the reduced group C^* -algebra C^*_r(\Gamma) and of the boundary crossed product algebra C(\partial X) \rtimes\Gamma with Hecke operators. In the case when \Gamma is a group of real hyperbolic isometries, the K -theory and K -homology groups of these C^{*} -algebras are related by a Gysin six-term exact sequence and we prove that this Gysin sequence is Hecke equivariant. Finally, when \Gamma is a Bianchi group, we assign explicit unbounded Fredholm modules (i.e. spectral triples) to (co)homology classes, inducing Hecke-equivariant isomorphisms between the integral cohomology of \Gamma and each of these K -groups. Our methods apply to case \Gamma \subset \mathbf {PSL}(\mathbf Z) as well. In particular we employ the unbounded Kasparov product to push the Dirac operator an embedded surface in the Borel–Serre compactification of \mathbf H/\Gamma to a spectral triple on the purely infinite geodesic boundary crossed product algebra C(\partial \mathbf H) \rtimes\Gamma .

Boundary and Eisenstein cohomology of mathrm {SL}_3({mathbb {Z}})

In this article, several cohomology spaces associated to the arithmetic groups mathrm {SL}_3({mathbb {Z}}) and mathrm {GL}_3({mathbb {Z}}) with coefficients in any highest weight representation {mathcal {M}}_lambda have been computed, where lambda denotes their highest weight. Consequently, we obtain detailed information of their Eisenstein cohomology with coefficients in {mathcal {M}}_lambda . When {mathcal {M}}_lambda is not self dual, the Eisenstein cohomology coincides with the cohomology of the underlying arithmetic group with coefficients in {mathcal {M}}_lambda . In particular, for such a large class of representations we can explicitly describe the cohomology of these two arithmetic groups. We accomplish this by studying the cohomology of the boundary of the Borel–Serre compactification and their Euler characteristic with coefficients in {mathcal {M}}_lambda . At the end, we employ our study to discuss the existence of ghost classes.

Hecke modules for arithmetic groups via bivariant K-theory

Let $\Gamma$ be a lattice in a locally compact group $G$. In earlier work, we used $KK$-theory to equip the $K$-groups of any $\Gamma$-$C^{*}$-algebra on which the commensurator of $\Gamma$ acts with Hecke operators. When $\Gamma$ is arithmetic, this gives Hecke operators on the $K$-theory of certain $C^{*}$-algebras that are naturally associated with $\Gamma$. In this paper, we first study the topological $K$-theory of the arithmetic manifold associated to $\Gamma$. We prove that the Chern character commutes with Hecke operators. Afterwards, we show that the Shimura product of double cosets naturally corresponds to the Kasparov product and thus that the $KK$-groups associated to an arithmetic group $\Gamma$ become true Hecke modules. We conclude by discussing Hecke equivariant maps in $KK$-theory in great generality and apply this to the Borel-Serre compactification as well as various noncommutative compactifications associated with $\Gamma$. Along the way we discuss the relation between the $K$-theory and the integral cohomology of low-dimensional manifolds as Hecke modules.

From the Borel–Serre compactification to curve complex of surfaces

From the Borel–Serre compactification to curve complex of surfaces

Motivic weightless complex and the relative Artin motive

We construct a motivic analogue of the weightless cohomology groups [18] and demonstrate several expected properties – ring structure, Kunneth, functoriality for certain morphisms etc. We also demonstrate the expected relationship with the motivic intersection complex of Wildeshaus [26,25] (whenever it exists). The methods also give another construction of the weightless cohomology groups over any base field. In characteristic 0, this motive coincides with the motive EX of Ayoub–Zucker [3] and recovers the motivic construction of the reductive Borel–Serre compactification of Shimura varieties.

A GLUING FORMULA FOR THE ANALYTIC TORSION ON HYPERBOLIC MANIFOLDS WITH CUSPS

For an odd-dimensional oriented hyperbolic manifold with cusps and strongly acyclic coefficient systems, we define the Reidemeister torsion of the Borel–Serre compactification of the manifold using bases of cohomology classes defined via Eisenstein series by the method of Harder. In the main result of this paper we relate this combinatorial torsion to the regularized analytic torsion. Together with results on the asymptotic behaviour of the regularized analytic torsion, established previously, this should have applications to study the growth of torsion in the cohomology of arithmetic groups. Our main result is established via a gluing formula, and here our approach is heavily inspired by a recent paper of Lesch.

L2–invariants of nonuniform lattices in semisimple Lie groups

We compute L2-invariants of certain nonuniform lattices in semisimple Lie groups by means of the Borel-Serre compactification of arithmetically defined locally symmetric spaces. The main results give new estimates for Novikov-Shubin numbers and vanishing L2-torsion for lattices in groups with even deficiency. We discuss applications to Gromov's Zero-in-the-Spectrum Conjecture as well as to a proportionality conjecture for the L2-torsion of measure equivalent groups.

The geometric theta correspondence for Hilbert modular surfaces

In a series of papers we have been studying the geometric theta correspondence for non-compact arithmetic quotients of symmetric spaces associated to orthogonal groups. It is our overall goal to develop a general theory of geometric theta liftings in the context of the real differential geometry/topology of non-compact locally symmetric spaces of orthogonal and unitary groups which generalizes the theory of Kudla-Millson in the compact case. In this paper we study in detail the geometric theta lift for Hilbert modular surfaces. In particular, we will give a new proof and an extension (to all finite index subgroups of the Hilbert modular group) of the celebrated theorem of Hirzebruch and Zagier that the generating function for the intersection numbers of the Hirzebruch-Zagier cycles is a classical modular form of weight 2. In our approach we replace Hirzebuch's smooth complex analytic compactification $\tilde{X}$ of the Hilbert modular surface $X$ with the (real) Borel-Serre compactification $\bar{X}$. The various algebro-geometric quantities are then replaced by topological quantities associated to 4-manifolds with boundary. In particular, the "boundary contribution" in Hirzebruch-Zagier is replaced by sums of linking numbers of circles (the boundaries of the cycles) in the 3-manifolds of type Sol (torus bundle over a circle) which comprise the Borel-Serre boundary.

On a question of Serre

Consider an imaginary quadratic number field Q(−m), with m a square-free positive integer, and its ring of integers O. The Bianchi groups are the groups SL2(O). Further consider the Borel–Serre compactification [7] (1970) of the quotient of hyperbolic 3-space H by a finite index subgroup Γ in a Bianchi group, and in particular the following question which Serre posed on p. 514 of the quoted article. Consider the map α induced on homology when attaching the boundary into the Borel–Serre compactification. How can one determine the kernel of α (in degree 1)? of the kernel of α. In the quoted article, Serre did add the question what submodule precisely this kernel is. Through a local topological study, we can decompose the kernel of α into its parts associated to each cusp.

Boundary behaviour of special cohomology classes arising from the Weil representation

AbstractIn our previous paper [J. Funke and J. Millson, Cycles with local coefficients for orthogonal groups and vector-valued Siegel modular forms, American J. Math. 128 (2006), 899–948], we established a correspondence between vector-valued holomorphic Siegel modular forms and cohomology with local coefficients for local symmetric spaces $X$ attached to real orthogonal groups of type $(p, q)$. This correspondence is realized using theta functions associated with explicitly constructed ‘special’ Schwartz forms. Furthermore, the theta functions give rise to generating series of certain ‘special cycles’ in $X$ with coefficients.In this paper, we study the boundary behaviour of these theta functions in the non-compact case and show that the theta functions extend to the Borel–Sere compactification $ \overline{X} $ of $X$. However, for the $ \mathbb{Q} $-split case for signature $(p, p)$, we have to construct and consider a slightly larger compactification, the ‘big’ Borel–Serre compactification. The restriction to each face of $ \overline{X} $ is again a theta series as in [J. Funke and J. Millson, loc. cit.], now for a smaller orthogonal group and a larger coefficient system.As an application we establish in certain cases the cohomological non-vanishing of the special (co)cycles when passing to an appropriate finite cover of $X$. In particular, the (co)homology groups in question do not vanish. We deduce as a consequence a sharp non-vanishing theorem for ${L}^{2} $-cohomology.

Simplicial volume of moduli spaces of Riemann surfaces

Motivated by results on the simplicial volume of locally symmetric spaces of finite volume, in this note, we observe that the simplicial volume of the moduli space $M_{g,n}$ is equal to $0$ if $g \ge 2$; $g = 1$, $n \ge 3$; or $g = 0$, $n \ge 6$; and the orbifold simplicial volume of $M_{g,n}$ is positive if $g = 1, n = 0, 1; g = 0, n = 4$. We also observe that the simplicial volume of the Deligne-Mumford compactification of $M_{g,n}$ is equal to $0$, and the simplicial volumes of the reductive Borel-Serre compactification of arithmetic locally symmetric spaces $\Gamma\backslash X$ and the Baily-Borel compactification of Hermitian arithmetic locally symmetric spaces $\Gamma\backslash X$ are also equal to $0$ if the $\mathbb{Q}$-rank of $\Gamma\backslash X$ is at least $3$ or if $\Gamma\backslash X$ is irreducible and of $\mathbb{Q}$-rank 2.

A twisted topological trace formula for Hecke operators and liftings from symplectic to general linear groups

AbstractFor the locally symmetric space X attached to an arithmetic subgroup of an algebraic group G of ℚ-rank r, we construct a compact manifold $\tilde X$ by gluing together 2r copies of the Borel–Serre compactification of X. We apply the classical Lefschetz fixed point formula to $\tilde X$ and get formulas for the traces of Hecke operators ℋ acting on the cohomology of X. We allow twistings of ℋ by outer automorphisms η of G. We stabilize this topological trace formula and compare it with the corresponding formula for an endoscopic group of the pair (G,η) . As an application, we deduce a weak lifting theorem for the lifting of automorphic representations from Siegel modular groups to general linear groups.

Relative Artin motives and the reductive Borel–Serre compactification of a locally symmetric variety

We introduce the notion of Artin motives and cohomological motives over a scheme X. Given a cohomological motive M over X, we construct its punctual weight zero part $\omega^0_X(M)$ as the universal Artin motive mapping to M. We use this to define a motive E_X over X which is an invariant of the singularities of X. The first half of the paper is devoted to the study of the functors $\omega^0_X$ and the computation of the motives E_X. In the second half of the paper, we develop the application to locally symmetric varieties. Specifically, let Y be a locally symmetric variety and denote by p:W-->Z the projection of its reductive Borel-Serre compactification W onto its Baily-Borel Satake compactification Z. We show that $Rp_*(\Q_W)$ is naturally isomorphic to the Betti realization of the motive E_Z, where Z is viewed as a scheme. In particular, the direct image of E_Z along the projection of Z to Spec(C) gives a motive whose Betti realization is naturally isomorphic to the cohomology of W.

The Asymptotic Schottky Problem

Let $${\mathcal{M}_g}$$ denote the moduli space of compact Riemann surfaces of genus g and let $${\mathcal{A}_g}$$ be the moduli space of principally polarized abelian varieties of dimension g. Let $${J : \mathcal{M}_g \rightarrow \mathcal{A}_g}$$ be the map which associates to a Riemann surface its Jacobian. The map J is injective, and the image $${\mathcal{J}_g := J(\mathcal{M}_g)}$$ is contained in a proper subvariety of $${\mathcal{A}_g}$$ when g ≥ 4. The classical and long-studied Schottky problem is to characterize the Jacobian locus $${\mathcal{J}_g}$$ in $${\mathcal{A}_g}$$ . In this paper we address a large scale version of this problem posed by Farb and called the coarse Schottky problem: What does $${\mathcal{J}_g}$$ look like “from far away”, or how “dense” is $${\mathcal{J}_g}$$ in the sense of coarse geometry? The large scale geometry of $${\mathcal{A}_g}$$ is encoded in its asymptotic cone, $${{\rm Cone}_\infty(\mathcal{A}_g)}$$ , which is a Euclidean simplicial cone of real dimension g. Our main result asserts that the Jacobian locus $${\mathcal{J}_g}$$ is “coarsely dense” in $${\mathcal{A}_g}$$ , which implies that the subset of $${{\rm Cone}_\infty(\mathcal{A}_g)}$$ determined by $${\mathcal{J}_g}$$ actually coincides with this cone. The proof shows that the Jacobian locus of hyperelliptic curves is coarsely dense in $${\mathcal{A}_g}$$ as well. We also study the boundary points of the Jacobian locus $${\mathcal{J}_g}$$ in $${\mathcal{A}_g}$$ and in the Baily–Borel and the Borel–Serre compactification. We show that for large genus g the set of boundary points of $${\mathcal{J}_g}$$ in these compactifications is “small”.

On the reductive Borel-Serre compactification, II: Excentric quotients and least common modifications

Let $X$ be a locally symmetric variety, i.e., the quotient of a bounded symmetric domain by a (say) neat arithmetically-defined group of isometries. Let ${\overline X}^{\rm exc}$ and $X^{{\rm tor,exc}}$ denote its excentric Borel-Serre and toroidal compactifications respectively. We determine their least common modification and use it to prove a conjecture of Goresky and Tai concerning canonical extensions of homogeneous vector bundles. In the process, we see that ${\overline X}^{\rm exc}$ and $X^{{\rm tor,exc}}$ are homotopy equivalent.