Transactions of the American Mathematical Society Preprint version available at

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Transactions of the American Mathematical Society 355 (2003), 4825-4846 Preprint version available at A GEOMETRIC CHARACTERIZATION OF VASSILIEV INVARIANTS MICHAEL EISERMANN Abstract. It is a well-known paradigm to consider Vassiliev invariants as polynomials on the set of knots. We prove the following characterization: a rational knot invariant is a Vassiliev invariant of degree ≤ m if and only if it is a polynomial of degree ≤ m on every geometric sequence of knots. Here a sequence Kz with z ? Z is called geometric if the knots Kz coincide outside a ball B, inside of which they satisfy Kz ?B = ?z for all z and some pure braid ? . As an application we show that the torsion in the braid group over the sphere induces torsion at the level of Vassiliev invariants: there exist knots in S1 ? S2 that can be distinguished by Z/2-invariants of finite type but not by rational invariants of finite type. In order to obtain such torsion invariants we construct over Z a universal Vassiliev invariant of degree 1 for knots in S1?S2. Introduction and statement of results A Vassiliev invariant is a map v : K ? A from the set of knots K to an abelian group A such that v satisfies a certain finiteness condition (see 1). Vassiliev invariants are commonly interpreted as polynomials on the set of knots [2, 3, 21].

• coefficients c?? ?


• vassiliev invariant


• zbn ?


• singular knots


• knot invariant


• trivial torsion


• torsion invariants


• zd ?


• geometric characterization

Sujets

• Finite type invariant
• Knot invariant

Informations

• Publié par: pefav
• Nombre de lectures: 10
• Langue: English

Extrait

Transactions of the American Mathematical Society 355 (2003), 4825-4846 Preprint version available at http://www-fourier.ujf-grenoble.fr/ ∼ eiserm

A GEOMETRIC CHARACTERIZATION OF VASSILIEV INVARIANTS

MICHAEL EISERMANN

Abstract.is a well-known paradigm to consider Vassiliev invariants asIt polynomials on the set of knots. We prove the following characterization: a rational knot invariant is a Vassiliev invariant of degree≤mif and only if it is a polynomial of degree≤m Hereon every geometric sequence of knots. a sequenceK z withz∈Zis calledgeometricif the knotsK z coincide outside a ballB, inside of which they satisfyK z ∩B=τ z for allzand some pure braid τthat the torsion in the braid group over the. As an application we show sphere induces torsion at the level of Vassiliev invariants: there exist knots in S 1 ×S 2 that can be distinguished byZ/2-invariants of finite type but not by rational invariants of finite type. In order to obtain such torsion invariants we construct overZa universal Vassiliev invariant of degree 1 for knots inS 1 ×S 2 .

Introduction and statement of results

A Vassiliev invariant is a mapv:K→Afrom the set of knotsKto an abelian groupAsuch thatvsatisfies a certain finiteness condition (see§1). Vassiliev invariants are commonly interpreted as polynomials on the set of knots [2,3,21]. One instance of this analogy is the following criterion:

Theorem [(J. Dean6], R. [ Trapp20]).A Vassiliev invariantv:K→Qof degree ≤mis a polynomial of degree≤mon every twist sequence of knots.

A twist sequence is a family of knotsK z (indexed byz∈Z) that are the same outside a ball, inside of which they differ as depicted in Figure1 this cri-. Using terion, J. Dean and R. Trapp showed that the class of Vassiliev invariants does not contain certain classical knot invariants such as crossing number, genus, signature, unknotting number, bridge number or braid index.

Figure 1.Local picture of a twist sequence of knots

Date: June 30, 2003 (revised version). 1991Mathematics Subject Classification.57M27, 57M25, 20F36. Key words and phrases.Vassiliev invariant, invariant of finite type, twist sequence, geometric sequence of knots, torsion in the braid group over the sphere, Dirac twist, Dirac’s spin trick. 1

2

MICHAEL EISERMANN

This article addresses the question of how toehtccraaezirVassiliev invariants by means of such geometric conditions. To begin with, twist sequences alone do not suffice, as shown in§5by means of a counterexample.

Geometric characterization.LetMbe a 3-manifold, and letKMbe its set of knots considered up to isotopy. Ageometric sequenceof knots is a sequenceK z indexed byz∈Zsuch that the knotsK z coincide outside a ballB⊂M, inside of which they satisfyK z ∩B=τ z for allzand some pure braidτ sequences. Such were introduced by T. Stanford [19] under the nametangle maps. We establish the following characterization of rational Vassiliev invariants.

Theorem 1(proved in§4).A mapv:KM→Qis a Vassiliev invariant of degree ≤mif and only if it is a polynomial of degree≤mon every geometric sequence.

Atwist sequenceis a special case of a geometric sequence whereτis a full twist of only two strands, as shown in Figure1. We explain in§5that twist sequences alone do not suffice to characterize Vassiliev invariants. For knots in the 3-sphere, however, we are led to the following characterization. Theorem 2(proved in§6).A mapv:KS 3 →Qis a Vassiliev invariant of degree ≤mif and only ifvis a polynomial on every twist sequence of knots and globally bounded by a polynomial of degreemin the crossing number.

For a knot invariant with values in a torsion group the characterization is less simple: the geometric sequence condition of Theorem1is necessary but perhaps not sufficient, and the boundedness condition of Theorem2cannot even be for- mulated. In this case a characterization can still be established using geometric lattices instead of sequences (see§3, Theorem28). The geometric characterization extends verbatim to Vassiliev invariants of links, tangles or embedded graphs. Generally speaking, these results give further evidence to the paradigm thatVassiliev invariants are polynomials.

Torsion invariants.Section9uses Dirac’s spin trick to construct an involutionθ on the set of knots in a reducible 3-manifold. Using geometric sequences, it is easy to see that rational Vassiliev invariants cannot distinguish a knotKfrom its twin knotθK the manifold. ForS 1 ×S 2 we prove: Theorem 3(proved in§§9–10).For every knotKinS 1 ×S 2 having homology class[K]∈ {±3±5±7 . . .}the following assertions hold: (1)The knotKand its twinθKare distinguished by a suitable Vassiliev in- variantK(S 1 ×S 2 )→Z/2of degree1. (2)The knotKand its twinθKcannot be distinguished by any Vassiliev in- variantK(S 1 ×S 2 )→Aif the abelian groupAhas no2-n.iorsto In particular, rational invariants of finite type do not distinguish all knots inS 1 ×S 2 .

Since this is the first occurrence of torsion in the Vassiliev theory of knots, we analyze this example in more detail. In Section10we carry out a combinatorial integration in order to construct aZ-universal Vassiliev invariant of degree 1: Theorem 4(proved in§10).LetM=S 1 ×S 2 , and letA 1 Mbe theZ-module of chord diagrams modulo the obvious 1T and Kirby relations (as explained in§10.2). Then there exists a universal Vassiliev invariantZ 1 M :KM→A 1 Mof degree1.