Danny Calegari, Notes on Agol's Virtual Haken Theorem
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On the Kinematic Formula in the Lives of the Saints Danny Calegari
SHORT STORIES On the Kinematic Formula in the Lives of the Saints Danny Calegari Saint Sebastian was an early Christian martyr. He served as a captain in the Praetorian Guard under Diocletian un- til his religious faith was discovered, at which point he was taken to a field, bound to a stake, and shot by archers “till he was as full of arrows as an urchin1 is full of pricks2.” Rather miraculously, he made a full recovery, but was later executed anyway for insulting the emperor. The trans- pierced saint became a popular subject for Renaissance painters, e.g., Figure 1. The arrows in Mantegna’s painting have apparently ar- rived from all directions, though they are conspicuously grouped around the legs and groin, almost completely missing the thorax. Intuitively, we should expect more ar- rows in the parts of the body that present a bigger cross section. This intuition is formalized by the claim that a subset of the surface of Saint Sebastian has an area pro- portional to its expected number of intersections with a random line (i.e., arrow). Since both area and expectation are additive, we may reduce the claim (by polygonal ap- proximation and limit) to the case of a flat triangular Saint Sebastian, in which case it is obvious. Danny Calegari is a professor of mathematics at the University of Chicago. His Figure 1. Andrea Mantegna’s painting of Saint Sebastian. email address is [email protected]. 1 hedgehog This is a 3-dimensional version of the classical Crofton 2quills formula, which says that the length of a plane curve is pro- For permission to reprint this article, please contact: portional to its expected number of intersections with a [email protected]. -
Groups of PL Homeomorphisms of Cubes
GROUPS OF PL HOMEOMORPHISMS OF CUBES DANNY CALEGARI AND DALE ROLFSEN D´edi´e`aMichel Boileau sur son soixanti`eme anniversaire. Sant´e! Abstract. We study algebraic properties of groups of PL or smooth homeo- morphisms of unit cubes in any dimension, fixed pointwise on the boundary, and more generally PL or smooth groups acting on manifolds and fixing point- wise a submanifold of codimension 1 (resp. codimension 2), and show that such groups are locally indicable (resp. circularly orderable). We also give many examples of interesting groups that can act, and discuss some other algebraic constraints that such groups must satisfy, including the fact that a group of PL homeomorphisms of the n-cube (fixed pointwise on the boundary) contains no elements that are more than exponentially distorted. 1. Introduction We are concerned in this paper with algebraic properties of the group of PL homeomorphisms of a PL manifold, fixed on some PL submanifold (usually of codimension 1 or 2, for instance the boundary) and some of its subgroups (usually those preserving some structure). The most important case is the group of PL homeomorphisms of In fixed pointwise on ∂In; hence these are “groups of PL homeomorphisms of the (n-)cube”. The algebraic study of transformation groups (often in low dimension, or preserv- ing some extra structure such as a symplectic or complex structure) has recently seen a lot of activity; however, much of this activity has been confined to the smooth category. It is striking that many of these results can be transplanted to the PL category. This interest is further strengthened by the possibility of working in the PL category over (real) algebraic rings or fields (this possibility has already been exploited in dimension 1, in the groups F and T of Richard Thompson). -
Commentary on Thurston's Work on Foliations
COMMENTARY ON FOLIATIONS* Quoting Thurston's definition of foliation [F11]. \Given a large supply of some sort of fabric, what kinds of manifolds can be made from it, in a way that the patterns match up along the seams? This is a very general question, which has been studied by diverse means in differential topology and differential geometry. ... A foliation is a manifold made out of striped fabric - with infintely thin stripes, having no space between them. The complete stripes, or leaves, of the foliation are submanifolds; if the leaves have codimension k, the foliation is called a codimension k foliation. In order that a manifold admit a codimension- k foliation, it must have a plane field of dimension (n − k)." Such a foliation is called an (n − k)-dimensional foliation. The first definitive result in the subject, the so called Frobenius integrability theorem [Fr], concerns a necessary and sufficient condition for a plane field to be the tangent field of a foliation. See [Spi] Chapter 6 for a modern treatment. As Frobenius himself notes [Sa], a first proof was given by Deahna [De]. While this work was published in 1840, it took another hundred years before a geometric/topological theory of foliations was introduced. This was pioneered by Ehresmann and Reeb in a series of Comptes Rendus papers starting with [ER] that was quickly followed by Reeb's foundational 1948 thesis [Re1]. See Haefliger [Ha4] for a detailed account of developments in this period. Reeb [Re1] himself notes that the 1-dimensional theory had already undergone considerable development through the work of Poincare [P], Bendixson [Be], Kaplan [Ka] and others. -
Arxiv:Math/0208110V1 [Math.GT] 14 Aug 2002 Disifiieymn Itntsraebnl Structures
STRONGLY IRREDUCIBLE SURFACE AUTOMORPHISMS SAUL SCHLEIMER Abstract. A surface automorphism is strongly irreducible if every essential simple closed curve in the surface has nontrivial geomet- ric intersection with its image. We show that a three-manifold admits only finitely many inequivalent surface bundle structures with strongly irreducible monodromy. 1. Introduction A surface automorphism h : F F is strongly irreducible if every es- sential simple closed curve γ F→has nontrivial geometric intersection with its image, h(γ). This paper⊂ shows that a three-manifold admits only finitely many inequivalent surface bundle structures with strongly irreducible monodromy. This imposes a serious restriction; for exam- ple, any three-manifold which fibres over the circle and has b2(M) 2 admits infinitely many distinct surface bundle structures. ≥ The main step is an elementary proof that all weakly acylindrical sur- faces inside of an irreducible triangulated manifold are isotopic to fun- damental normal surfaces. As weakly acylindrical surfaces are a larger class than the acylindrical surfaces this strengthens a result of Hass [5]; an irreducible three-manifold contains only finitely many acylindrical surfaces. Section 2 gives necessary topological definitions, examples of strongly irreducible automorphisms, and precise statements of the theorems. arXiv:math/0208110v1 [math.GT] 14 Aug 2002 The required tools of normal surface theory are presented in Section 3. Section 4 defines weakly acylindrical and proves that every such sur- face is isotopic to a fundamental surface. In the spirit of the Georgia Topology Conference the paper ends by listing several open questions. Many of the ideas and terminology discussed come from the study of Heegaard splittings as in [2] and in my thesis [11]. -
Lecture Notes C Sarah Rasmussen, 2019
Part III 3-manifolds Lecture Notes c Sarah Rasmussen, 2019 Contents Lecture 0 (not lectured): Preliminaries2 Lecture 1: Why not ≥ 5?9 Lecture 2: Why 3-manifolds? + Introduction to knots and embeddings 13 Lecture 3: Link diagrams and Alexander polynomial skein relations 17 Lecture 4: Handle decompositions from Morse critical points 20 Lecture 5: Handles as Cells; Morse functions from handle decompositions 24 Lecture 6: Handle-bodies and Heegaard diagrams 28 Lecture 7: Fundamental group presentations from Heegaard diagrams 36 Lecture 8: Alexander polynomials from fundamental groups 39 Lecture 9: Fox calculus 43 Lecture 10: Dehn presentations and Kauffman states 48 Lecture 11: Mapping tori and Mapping Class Groups 54 Lecture 12: Nielsen-Thurston classification for mapping class groups 58 Lecture 13: Dehn filling 61 Lecture 14: Dehn surgery 64 Lecture 15: 3-manifolds from Dehn surgery 68 Lecture 16: Seifert fibered spaces 72 Lecture 17: Hyperbolic manifolds 76 Lecture 18: Embedded surface representatives 80 Lecture 19: Incompressible and essential surfaces 83 Lecture 20: Connected sum 86 Lecture 21: JSJ decomposition and geometrization 89 Lecture 22: Turaev torsion and knot decompositions 92 Lecture 23: Foliations 96 Lecture 24. Taut Foliations 98 Errata: Catalogue of errors/changes/addenda 102 References 106 1 2 Lecture 0 (not lectured): Preliminaries 0. Notation and conventions. Notation. @X { (the manifold given by) the boundary of X, for X a manifold with boundary. th @iX { the i connected component of @X. ν(X) { a tubular (or collared) neighborhood of X in Y , for an embedding X ⊂ Y . ◦ ν(X) { the interior of ν(X). This notation is somewhat redundant, but emphasises openness. -
Counting Essential Surfaces in 3-Manifolds
Counting essential surfaces in 3-manifolds Nathan M. Dunfield, Stavros Garoufalidis, and J. Hyam Rubinstein Abstract. We consider the natural problem of counting isotopy classes of es- sential surfaces in 3-manifolds, focusing on closed essential surfaces in a broad class of hyperbolic 3-manifolds. Our main result is that the count of (possibly disconnected) essential surfaces in terms of their Euler characteristic always has a short generating function and hence has quasi-polynomial behavior. This gives remarkably concise formulae for the number of such surfaces, as well as detailed asymptotics. We give algorithms that allow us to compute these generating func- tions and the underlying surfaces, and apply these to almost 60,000 manifolds, providing a wealth of data about them. We use this data to explore the delicate question of counting only connected essential surfaces and propose some con- jectures. Our methods involve normal and almost normal surfaces, especially the work of Tollefson and Oertel, combined with techniques pioneered by Ehrhart for counting lattice points in polyhedra with rational vertices. We also introduce arXiv:2007.10053v1 [math.GT] 20 Jul 2020 a new way of testing if a normal surface in an ideal triangulation is essential that avoids cutting the manifold open along the surface; rather, we use almost normal surfaces in the original triangulation. Contents 1 Introduction3 1.2 Main results . .4 1.5 Motivation and broader context . .5 1.7 The key ideas behind Theorem 1.3......................6 2 1.8 Making Theorem 1.3 algorithmic . .8 1.9 Ideal triangulations and almost normal surfaces . .8 1.10 Computations and patterns . -
Universal Circles for Quasigeodesic Flows 1 Introduction
Geometry & Topology 10 (2006) 2271–2298 2271 arXiv version: fonts, pagination and layout may vary from GT published version Universal circles for quasigeodesic flows DANNY CALEGARI We show that if M is a hyperbolic 3–manifold which admits a quasigeodesic flow, then π1(M) acts faithfully on a universal circle by homeomorphisms, and preserves a pair of invariant laminations of this circle. As a corollary, we show that the Thurston norm can be characterized by quasigeodesic flows, thereby generalizing a theorem of Mosher, and we give the first example of a closed hyperbolic 3–manifold without a quasigeodesic flow, answering a long-standing question of Thurston. 57R30; 37C10, 37D40, 53C23, 57M50 1 Introduction 1.1 Motivation and background Hyperbolic 3–manifolds can be studied from many different perspectives. A very fruitful perspective is to think of such manifolds as dynamical objects. For example, a very important class of hyperbolic 3–manifolds are those arising as mapping tori of pseudo-Anosov automorphisms of surfaces (Thurston [24]). Such mapping tori naturally come with a flow, the suspension flow of the automorphism. In the seminal paper [4], Cannon and Thurston showed that this suspension flow can be chosen to be quasigeodesic and pseudo-Anosov. Informally, a flow on a hyperbolic 3–manifold is quasigeodesic if the lifted flowlines in the universal cover are quasi-geodesics in H3 , and a flow is pseudo-Anosov if it looks locally like a semi-branched cover of an Anosov flow. Of course, such a flow need not be transverse to a foliation by surfaces. See Thurston [24] and Fenley [6] for background and definitions. -
Arxiv:2101.02945V3 [Math.GT] 15 Apr 2021
On Crossing Ball Structure in Knot and Link Complements Wei Lin Abstract We develop a word mechanism applied in knot and link diagrams for the illustra- tion of a diagrammatic property. We also give a necessary condition for determining incompressible and pairwise incompressible surfaces, that are embedded in knot or link complements. Finally, we give a finiteness theorem and an upper bound on the Euler characteristic of such surfaces. 1 Introduction 1.1 Preliminary Discussion Let L ⊂ S3 be a link and π(L) ⊂ S2(⊂ S3) be a regular link projection. Additionally, let F ⊂ S3 − L be an closed incompressible surface. In 1981 Menasco introduced his crossing ball technology for classical link projections [7] that replaced π(L) in S2 with two 2-spheres, 2 2 2 3 2 2 S±, which had the salient features that L was embedded in S+ [ S− and S n (S+ [ S−) 3 2 was a collection of open 3-balls|B± that correspond to the boundaries S± and a collect of crossing balls. (Please see §1.2 for formal definition.) Using general position arguments, 2 2 this technology allows for placing F into normal position with respect to S± so that F \S± is a collection of simple closed curves (s.c.c.'s). When one imposes the assumption that π(L) is an alternating projection, the normal position of an essential surface is exceedingly well behaved to the point where by direct observation one can definitively state whether the link is split, prime, cabled or a satellite. As such, any alternating knot can by direct observation be placed into one of William Thurston's three categories|torus knot, satellite knot or hyperbolic knot [10]. -
Extension of Incompressible Surfaces on the Boundaries of 3-Manifolds
PACIFIC JOURNAL OF MATHEMATICS Volume 194 (2000), pages 335{348 EXTENSION OF INCOMPRESSIBLE SURFACES ON THE BOUNDARIES OF 3-MANIFOLDS Michael Freedman1, Hugh Howards and Ying-Qing Wu2 An incompressible bounded surface F on the boundary of a compact, connected, orientable 3-manifold M is arc-extendible if there is a properly embedded arc γ on @M − IntF such that F [ N(γ) is incompressible, where N(γ) is a regular neighbor- hood of γ in @M. Suppose for simplicity that M is irreducible and F has no disk components. If M is a product F × I, or if @M − F is a set of annuli, then clearly F is not arc-extendible. The main theorem of this paper shows that these are the only obstructions for F to be arc-extendible. Suppose F is a compact incompressible surface on the boundary of a compact, connected, orientable, irreducible 3-manifold M. Let F 0 be a component of @M − IntF with @F 0 6= ;. We say that F is arc-extendible (in F 0) if there is a properly embedded arc γ in F 0 such that F [ N(γ) is incompressible. In this case γ is called an extension arc of F . We study the problem of which incompressible surfaces on the boundary M are arc-extendible. The result will be used in [H], in which it is shown that a compact, orientable 3-manifold M contains arbitrarily many disjoint, non-parallel, non-boundary parallel, incompressible surfaces, if and only if M has at least one boundary component of genus greater than or equal to two. -
Department of Mathematics 1
Department of Mathematics 1 Department of Mathematics Chair • Shmuel Weinberger Professors • Laszlo Babai, Computer Science and Mathematics • Guillaume Bal, Statistics and Mathematics • Alexander A. Beilinson • Danny Calegari • Francesco Calegari • Kevin Corlette • Marianna Csörnyei • Vladimir Drinfeld • Todd Dupont, Computer Science and Mathematics • Matthew Emerton • Alex Eskin • Benson Farb • Robert A. Fefferman • Victor Ginzburg • Denis Hirschfeldt • Kazuya Kato • Carlos E. Kenig • Gregory Lawler, Mathematics and Statistics • Maryanthe Malliaris • J. Peter May • Andre Neves • Bao Châu Ngô • Madhav Vithal Nori • Alexander Razborov, Mathematics and Computer Science • Luis Silvestre • Charles Smart • Panagiotis Souganidis • Sidney Webster • Shmuel Weinberger • Amie Wilkinson • Robert Zimmer Associate Professors • Simion Filip • Ewain Gwynne Assistant Professors • Sebastian Hurtado-Salazar • Dana Mendelson • Nikita Rozenblyum • Daniil Rudenko Instructors • Lucas Benigni • Guher Camliyurt • Stephen Cantrell • Elliot Cartee • Mark Cerenzia 2 Department of Mathematics • Andrea Dotto • Mikolaj Fraczyk • Pedro Gasper • Kornelia Hera • Trevor Hyde • Kasia Jankiewicz • Justin Lanier • Brian Lawrence • Zhilin Luo • Akhil Mathew • Henrik Matthieson • Cornelia Mihaila • Lucia Mocz • Benedict Morrissey • Davi Obata • Lue Pan • Wenyu Pan • Beniada Shabani • Danny Shi • Daniel Stern • Ao Sun • Xuan Wu • Zihui Zhao • Jinping Zhuge Senior Lecturers • John Boller • Lucas Culler • Jitka Stehnova • Sarah Ziesler Lecturer • Meghan Anderson Assistant Instructional -
Arxiv:1911.02470V3 [Math.GT] 20 Apr 2021 Plicial Volume of Gr As Kgrk := Kαr,Rk1, the L -Semi-Norm of the Fundamental Class Αr (Section 3.1)
Simplicial volume of one-relator groups and stable commutator length Nicolaus Heuer, Clara L¨oh April 21, 2021 Abstract A one-relator group is a group Gr that admits a presentation hS j ri with a single relation r. One-relator groups form a rich classically studied class of groups in Geometric Group Theory. If r 2 F (S)0, we introduce a simplicial volume kGrk for one-relator groups. We relate this invariant to the stable commutator length sclS (r) of the element r 2 F (S). We show that often (though not always) the linear relationship kGrk = 4·sclS (r)−2 holds and that every rational number modulo 1 is the simplicial volume of a one-relator group. Moreover, we show that this relationship holds approximately for proper powers and for elements satisfying the small cancellation condition C0(1=N), with a multiplicative error of O(1=N). This allows us to prove for random 0 elements of F (S) of length n that kGrk is 2 log(2jSj − 1)=3 · n= log(n) + o(n= log(n)) with high probability, using an analogous result of Calegari{ Walker for stable commutator length. 1 Introduction A one-relator group is a group Gr that admits a presentation hS j ri with a single relation r 2 F (S). This rich and well studied class of groups in Geometric Group Theory generalises surface groups and shares many properties with them. A common theme is to relate the geometric properties of a classifying space of Gr to the algebraic properties of the relator r 2 F (S). -
Arxiv:Math/0605354V4 [Math.GT] 6 Jun 2007 Oino Hti En O Ojgc Ls Ohv Ml Tbeco Stable Small Have to Class Conjugacy a for Means It Length
LENGTH AND STABLE LENGTH DANNY CALEGARI Abstract. This paper establishes the existence of a gap for the stable length spectrum on a hyperbolic manifold. If M is a hyperbolic n-manifold, for every positive ǫ there is a positive δ depending only on n and on ǫ such that an element of π1(M) with stable commutator length less than δ is represented by a geodesic with length less than ǫ. Moreover, for any such M, the first accumulation point for stable commutator length on conjugacy classes is at least 1/12. Conversely, “most” short geodesics in hyperbolic 3-manifolds have arbi- trarily small stable commutator length. Thus stable commutator length is typically good at detecting the thick-thin decomposition of M, and 1/12 can be thought of as a kind of homological Margulis constant. 1. Introduction If M is a closed hyperbolic n-manifold with n> 2, the Mostow Rigidity Theorem says that the geometric structure on M is uniquely determined by the algebraic structure of π1(M), up to isometry. This means that in principle, any kind of geometric information about M (e.g. length spectrum, volume, systoles, etc.) can be recovered from group theory. On the other hand, with some notable exceptions, it is not easy to find concepts which have a straightforward interpretation on both the geometric and the algebraic side. One of the most important geometric structures on a hyperbolic manifold is the thick-thin decomposition. There is a universal positive constant ǫ(n) in each dimension (i.e. the Margulis constant; see [16]) such that the part of a hyperbolic n-manifold M with injectivity radius less than ǫ (i.e.