Dynamics, and Invariants
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An Introduction to Knot Theory and the Knot Group
AN INTRODUCTION TO KNOT THEORY AND THE KNOT GROUP LARSEN LINOV Abstract. This paper for the University of Chicago Math REU is an expos- itory introduction to knot theory. In the first section, definitions are given for knots and for fundamental concepts and examples in knot theory, and motivation is given for the second section. The second section applies the fun- damental group from algebraic topology to knots as a means to approach the basic problem of knot theory, and several important examples are given as well as a general method of computation for knot diagrams. This paper assumes knowledge in basic algebraic and general topology as well as group theory. Contents 1. Knots and Links 1 1.1. Examples of Knots 2 1.2. Links 3 1.3. Knot Invariants 4 2. Knot Groups and the Wirtinger Presentation 5 2.1. Preliminary Examples 5 2.2. The Wirtinger Presentation 6 2.3. Knot Groups for Torus Knots 9 Acknowledgements 10 References 10 1. Knots and Links We open with a definition: Definition 1.1. A knot is an embedding of the circle S1 in R3. The intuitive meaning behind a knot can be directly discerned from its name, as can the motivation for the concept. A mathematical knot is just like a knot of string in the real world, except that it has no thickness, is fixed in space, and most importantly forms a closed loop, without any loose ends. For mathematical con- venience, R3 in the definition is often replaced with its one-point compactification S3. Of course, knots in the real world are not fixed in space, and there is no interesting difference between, say, two knots that differ only by a translation. -
Knot Theory and the Alexander Polynomial
Knot Theory and the Alexander Polynomial Reagin Taylor McNeill Submitted to the Department of Mathematics of Smith College in partial fulfillment of the requirements for the degree of Bachelor of Arts with Honors Elizabeth Denne, Faculty Advisor April 15, 2008 i Acknowledgments First and foremost I would like to thank Elizabeth Denne for her guidance through this project. Her endless help and high expectations brought this project to where it stands. I would Like to thank David Cohen for his support thoughout this project and through- out my mathematical career. His humor, skepticism and advice is surely worth the $.25 fee. I would also like to thank my professors, peers, housemates, and friends, particularly Kelsey Hattam and Katy Gerecht, for supporting me throughout the year, and especially for tolerating my temporary insanity during the final weeks of writing. Contents 1 Introduction 1 2 Defining Knots and Links 3 2.1 KnotDiagramsandKnotEquivalence . ... 3 2.2 Links, Orientation, and Connected Sum . ..... 8 3 Seifert Surfaces and Knot Genus 12 3.1 SeifertSurfaces ................................. 12 3.2 Surgery ...................................... 14 3.3 Knot Genus and Factorization . 16 3.4 Linkingnumber.................................. 17 3.5 Homology ..................................... 19 3.6 TheSeifertMatrix ................................ 21 3.7 TheAlexanderPolynomial. 27 4 Resolving Trees 31 4.1 Resolving Trees and the Conway Polynomial . ..... 31 4.2 TheAlexanderPolynomial. 34 5 Algebraic and Topological Tools 36 5.1 FreeGroupsandQuotients . 36 5.2 TheFundamentalGroup. .. .. .. .. .. .. .. .. 40 ii iii 6 Knot Groups 49 6.1 TwoPresentations ................................ 49 6.2 The Fundamental Group of the Knot Complement . 54 7 The Fox Calculus and Alexander Ideals 59 7.1 TheFreeCalculus ............................... -
How Can We Say 2 Knots Are Not the Same?
How can we say 2 knots are not the same? SHRUTHI SRIDHAR What’s a knot? A knot is a smooth embedding of the circle S1 in IR3. A link is a smooth embedding of the disjoint union of more than one circle Intuitively, it’s a string knotted up with ends joined up. We represent it on a plane using curves and ‘crossings’. The unknot A ‘figure-8’ knot A ‘wild’ knot (not a knot for us) Hopf Link Two knots or links are the same if they have an ambient isotopy between them. Representing a knot Knots are represented on the plane with strands and crossings where 2 strands cross. We call this picture a knot diagram. Knots can have more than one representation. Reidemeister moves Operations on knot diagrams that don’t change the knot or link Reidemeister moves Theorem: (Reidemeister 1926) Two knot diagrams are of the same knot if and only if one can be obtained from the other through a series of Reidemeister moves. Crossing Number The minimum number of crossings required to represent a knot or link is called its crossing number. Knots arranged by crossing number: Knot Invariants A knot/link invariant is a property of a knot/link that is independent of representation. Trivial Examples: • Crossing number • Knot Representations / ~ where 2 representations are equivalent via Reidemester moves Tricolorability We say a knot is tricolorable if the strands in any projection can be colored with 3 colors such that every crossing has 1 or 3 colors and or the coloring uses more than one color. -
Altering the Trefoil Knot
Altering the Trefoil Knot Spencer Shortt Georgia College December 19, 2018 Abstract A mathematical knot K is defined to be a topological imbedding of the circle into the 3-dimensional Euclidean space. Conceptually, a knot can be pictured as knotted shoe lace with both ends glued together. Two knots are said to be equivalent if they can be continuously deformed into each other. Different knots have been tabulated throughout history, and there are many techniques used to show if two knots are equivalent or not. The knot group is defined to be the fundamental group of the knot complement in the 3-dimensional Euclidean space. It is known that equivalent knots have isomorphic knot groups, although the converse is not necessarily true. This research investigates how piercing the space with a line changes the trefoil knot group based on different positions of the line with respect to the knot. This study draws comparisons between the fundamental groups of the altered knot complement space and the complement of the trefoil knot linked with the unknot. 1 Contents 1 Introduction to Concepts in Knot Theory 3 1.1 What is a Knot? . .3 1.2 Rolfsen Knot Tables . .4 1.3 Links . .5 1.4 Knot Composition . .6 1.5 Unknotting Number . .6 2 Relevant Mathematics 7 2.1 Continuity, Homeomorphisms, and Topological Imbeddings . .7 2.2 Paths and Path Homotopy . .7 2.3 Product Operation . .8 2.4 Fundamental Groups . .9 2.5 Induced Homomorphisms . .9 2.6 Deformation Retracts . 10 2.7 Generators . 10 2.8 The Seifert-van Kampen Theorem . -
Knot Group Epimorphisms DANIEL S
Knot Group Epimorphisms DANIEL S. SILVER and WILBUR WHITTEN Abstract: Let G be a finitely generated group, and let λ ∈ G. If there 3 exists a knot k such that πk = π1(S \k) can be mapped onto G sending the longitude to λ, then there exists infinitely many distinct prime knots with the property. Consequently, if πk is the group of any knot (possibly composite), then there exists an infinite number of prime knots k1, k2, ··· and epimorphisms · · · → πk2 → πk1 → πk each perserving peripheral structures. Properties of a related partial order on knots are discussed. 1. Introduction. Suppose that φ : G1 → G2 is an epimorphism of knot groups preserving peripheral structure (see §2). We are motivated by the following questions. Question 1.1. If G1 is the group of a prime knot, can G2 be other than G1 or Z? Question 1.2. If G2 can be something else, can it be the group of a composite knot? Since the group of a composite knot is an amalgamated product of the groups of the factor knots, one might expect the answer to Question 1.1 to be no. Surprisingly, the answer to both questions is yes, as we will see in §2. These considerations suggest a natural partial ordering on knots: k1 ≥ k2 if the group of k1 maps onto the group of k2 preserving peripheral structure. We study the relation in §3. 2. Main result. As usual a knot is the image of a smooth embedding of a circle in S3. Two knots are equivalent if they have the same knot type, that is, there exists an autohomeomorphism of S3 taking one knot to the other. -
Notation and Basic Facts in Knot Theory
Appendix A Notation and Basic Facts in Knot Theory In this appendix, our aim is to provide a quick review of basic terminology and some facts in knot theory, which we use or need in this book. This appendix lists them item by item (without proof); so we do no attempt to give a full rigorous treatments; Instead, we work somewhat intuitively. The reader with interest in the details could consult the textbooks [BZ, R, Lic, KawBook]. • First, we fix notation on the circle S1 := {(x, y) ∈ R2 | x2 + y2 = 1}, and consider a finite disjoint union S1. A link is a C∞-embedding of L :S1 → S3 in the 3-sphere. We denote often by #L the number of the disjoint union, and denote the image Im(L) by only L for short. If #L is 1, L is usually called a knot, and is written K instead. This book discusses embeddings together with orientation. For an oriented link L, we denote by −L the link with its orientation reversed, and by L∗ the mirror image of L. • (Notations of link components). Given a link L :S1 → S3, let us fix an open tubular neighborhood νL ⊂ S3. Throughout this book, we denote the complement S3 \ νL by S3 \ L for short. Since we mainly discuss isotopy classes of S3 \ L,we may ignore the choice of νL. 2 • For example, for integers s, t ∈ Z , the torus link Ts,t (of type (s, t)) is defined by 3 2 s t 2 2 S Ts,t := (z,w)∈ C z + w = 0, |z| +|w| = 1 . -
The Ways I Know How to Define the Alexander Polynomial
ALL THE WAYS I KNOW HOW TO DEFINE THE ALEXANDER POLYNOMIAL KYLE A. MILLER Abstract. These began as notes for a talk given at the Student 3-manifold seminar, Spring 2019. There seems to be many ways to define the Alexander polynomial, all of which are somehow interrelated, but sometimes there is not an obvious path between any two given definitions. As the title suggests, this is an exploration of all the ways I know how to define this knot invariant. While we will touch on a number of facts and properties, these notes are not meant to be a complete survey of the Alexander polynomial. Contents 1. Introduction2 2. Alexander’s definition2 2.1. The Dehn presentation2 2.2. Abelianization4 2.3. The associated matrix4 3. The Alexander modules6 3.1. Orders7 3.2. Elementary ideals8 3.3. The Fox calculus 10 3.4. Torus knots 13 3.5. The Wirtinger presentation 13 3.6. Generalization to links 15 3.7. Fibered knots 15 3.8. Seifert presentation 16 3.9. Duality 18 4. The Conway potential 18 4.1. Alexander’s three-term relation 20 5. The HOMFLY-PT polynomial 24 6. Kauffman state sum 26 6.1. Another state sum 29 7. The Burau representation 29 8. Vassiliev invariants 29 9. The Alexander quandle 29 9.1. Projective Alexander quandles 33 10. Reidemeister torsion 33 11. Knot Floer homology 33 References 33 Date: May 3, 2019. 1 DRAFT 2020/11/14 18:57:28 2 KYLE A. MILLER 1. Introduction Recall that a link is an embedded closed 1-manifold in S3, and a knot is a 1-component link. -
Lectures Notes on Knot Theory
Lectures notes on knot theory Andrew Berger (instructor Chris Gerig) Spring 2016 1 Contents 1 Disclaimer 4 2 1/19/16: Introduction + Motivation 5 3 1/21/16: 2nd class 6 3.1 Logistical things . 6 3.2 Minimal introduction to point-set topology . 6 3.3 Equivalence of knots . 7 3.4 Reidemeister moves . 7 4 1/26/16: recap of the last lecture 10 4.1 Recap of last lecture . 10 4.2 Intro to knot complement . 10 4.3 Hard Unknots . 10 5 1/28/16 12 5.1 Logistical things . 12 5.2 Question from last time . 12 5.3 Connect sum operation, knot cancelling, prime knots . 12 6 2/2/16 14 6.1 Orientations . 14 6.2 Linking number . 14 7 2/4/16 15 7.1 Logistical things . 15 7.2 Seifert Surfaces . 15 7.3 Intro to research . 16 8 2/9/16 { The trefoil is knotted 17 8.1 The trefoil is not the unknot . 17 8.2 Braids . 17 8.2.1 The braid group . 17 9 2/11: Coloring 18 9.1 Logistical happenings . 18 9.2 (Tri)Colorings . 18 10 2/16: π1 19 10.1 Logistical things . 19 10.2 Crash course on the fundamental group . 19 11 2/18: Wirtinger presentation 21 2 12 2/23: POLYNOMIALS 22 12.1 Kauffman bracket polynomial . 22 12.2 Provoked questions . 23 13 2/25 24 13.1 Axioms of the Jones polynomial . 24 13.2 Uniqueness of Jones polynomial . 24 13.3 Just how sensitive is the Jones polynomial . -
Applied Topology Methods in Knot Theory
Applied topology methods in knot theory Radmila Sazdanovic NC State University Optimal Transport, Topological Data Analysis and Applications to Shape and Machine Learning//Mathematical Bisciences Institute OSU 29 July 2020 big data tools in pure mathematics Joint work with P. Dlotko, J. Levitt. M. Hajij • How to vectorize data, how to get a point cloud out of the shapes? Idea: Use a vector consisting of numerical descriptors • Methods: • Machine learning: How to use ML with infinite data that is hard to sample in a reasonable way. a • Topological data analysis • Hybrid between TDA and statistics: PCA combined with appropriate filtration • Goals: improving identification and circumventing obstacles from computational complexity of shape descriptors. big data tools in knot theory • Input: point clouds obtained from knot invariants • Machine learning • PCA Principal Component Analysis with different filtrations • Topological data analysis • Goals: • characterizing discriminative power of knot invariants for knot detection • comparing knot invariants • experimental evidence for conjectures knots and links 1 3 Knot is an equivalence class of smooth embeddings f : S ! R up to ambient isotopy. knot theory • Problems are easy to state but a remarkable breath of techniques are employed in answering these questions • combinatorial • algebraic • geometric • Knots are interesting on their own but they also provide information about 3- and 4-dimensional manifolds Theorem (Lickorish-Wallace) Every closed, connected, oriented 3-manifold can be obtained by doing surgery on a link in S3: knots are `big data' #C 0 3 4 5 6 7 8 9 10 #PK 1 1 1 2 3 7 21 49 165 #C 11 12 13 14 15 #PK 552 2,176 9,988 46,972 253,293 #C 16 17 18 19 #PK 1,388,705 8,053,393 48,266,466 294,130,458 Table: Number of prime knots for a given crossing number. -
The Combinatorial Revolution in Knot Theory Sam Nelson
The Combinatorial Revolution in Knot Theory Sam Nelson not theory is usually understood to be the study of embeddings of topologi- cal spaces in other topological spaces. Classical knot theory, in particular, is concerned with the ways in which a cir- Kcle or a disjoint union of circles can be embedded in R3. Knots are usually described via knot dia- grams, projections of the knot onto a plane with Figure 1. Knot diagrams. breaks at crossing points to indicate which strand passes over and which passes under, as in Fig- ure 1. However, much as the concept of “numbers” seriously, however, has led to the discovery of has evolved over time from its original meaning new types of generalized knots and links that do of cardinalities of finite sets to include ratios, not correspond to simple closed curves in R3. equivalence classes of rational Cauchy sequences, Like the complex numbers arising from missing roots of polynomials, and more, the classical con- roots of real polynomials, the new generalized cept of “knots” has recently undergone its own knot types appear as abstract solutions in knot evolutionary generalization. Instead of thinking equations that have no solutions among the classi- of knots topologically as ambient isotopy classes cal geometric knots. Although they seem esoteric of embedded circles or geometrically as simple at first, these generalized knots turn out to have R3 closed curves in , a new approach defines knots interpretations such as knotted circles or graphs combinatorially as equivalence classes of knot di- in three-manifolds other than R3, circuit diagrams, agrams under an equivalence relation determined and operators in exotic algebras. -
Knots and Links
Knots and Links H.R.Morton Spring 2002 1 Knots and Links 1 2001-02 1 Introduction We are all able to tie a knot in a piece of rope. What exactly do we mean though when we say that a piece of rope is knotted? Look at the pictures in figure 1. Figure 1: Let us first stop the knots escaping by joining up the ends of the rope, as in figure 2. Compare what happens in the three cases. In the first case we get a simple, or ‘unknotted’ circle, while in the second case we have a circle with what appears to be a knot in it. Let us say that the rope is knotted if no possible manipulation of it will result in the unknotted circle. We do not allow cutting and rejoining. The third example can clearly be undone by a little manipulation to form the simple circle, so again the rope is unknotted. 2 Knots and Links 1 2001-02 Figure 2: We model this notion of a knot mathematically by referring to a closed curve in R3 as a knot, with the special case of the simple circle, lying say as the unit circle in a plane, known as the trivial knot or unknot. Knot theory in the mathematical sense is then the study of closed curves in space. We call two knots equivalent if one can be manipulated, without passing one strand through another, to become the other knot. I give a more formal technical description of this below, but essentially anything is allowed which could be done with a rather stretchable piece of rope. -
Mutation and the Colored Jones Polynomial
Journal of G¨okova Geometry Topology Volume 3 (2009) 44 – 78 Mutation and the colored Jones polynomial Alexander Stoimenow and Toshifumi Tanaka with appendices by Daniel Matei and the first author Abstract. It is known that the colored Jones polynomials, various 2-cable link polynomials, the hyperbolic volume, and the fundamental group of the double branched cover coincide on mutant knots. We construct examples showing that these criteria, even in various combinations, are not sufficient to determine the mutation class of a knot, and that they are independent in several ways. In particular, we answer nega- tively the question of whether the colored Jones polynomial determines a simple knot up to mutation. 1. Introduction Mutation, introduced by Conway [Co], is a procedure of turning a knot into another, often a different but a “similar” one. This similarity alludes to the circumstance, that most of the common (efficiently computable) invariants coincide on mutants, and so mutants are difficult to distinguish. A basic exercise in skein theory shows that mutants have the same Alexander polynomial ∆, and this argument extends to the later discovered Jones V , HOMFLY(-PT or skein) P , BLMH Q and Kauffman F polynomials [J, F&, LM, BLM, Ho, Ka]. The cabling formula for the Alexander polynomial (see for example [Li, theorem 6.15]) shows also that Alexander polynomials of all satellite knots of mutants coincide, and the same was proved by Lickorish and Lipson [LL] also for the HOMFLY and Kauffman polynomials of 2-satellites of mutants. The HOMFLY polynomial applied on a 3-cable can generally distinguish mutants (for example the K-T and Conway knot; see 3.2), but with a calculation effort that is too large to be considered widely practicable.