Seifert circles,braid index and the algebraic crossing number

We introduce new operations reducing the number of Seifert circles in link diagrams of a special type. The operations are similar to one described in [Mem. Amer. Math. Soc. 508 (1993)] and [Math. Proc. Cambridge Philos. Soc. 111 (2) (1992) 273]. We discuss a conjecture about the number of Seifert circles that can be canceled by applying the operation repeatedly. We translate the problem into one belonging to graph theory.     

A formula for the braid index of links

Morton and Franks–Williams independently gave a lower bound for the braid index b(L) of a link L in S³ in terms of the v-span of the Homfly-pt polynomial P_L(v,z) of L: . Up to now, many classes of knots and links satisfying the equality of this Morton–Franks–Williams's inequality have been founded. In this paper, we give a new such a class of knots and links and make an explicit formula for determining the braid index of knots and links that belong to the class . This gives simultaneously a new class of knots and links satisfying the Jones conjecture which says that the algebraic crossing number in a minimal braid representation is a link invariant. We also give an algorithm to find a minimal braid representative for a given knot or link in .     

Generalized Seifert surfaces and signatures of colored links

In this paper, we use `generalized Seifert surfaces' to extend the Levine-Tristram signature to colored links in . This yields an integral valued function on the -dimensional torus, where is the number of colors of the link. The case corresponds to the Levine-Tristram signature. We show that many remarkable properties of the latter invariant extend to this -variable generalization: it vanishes for achiral colored links, it is `piecewise continuous', and the places of the jumps are determined by the Alexander invariants of the colored link. Using a -dimensional interpretation and the Atiyah-Singer -signature theorem, we also prove that this signature is invariant by colored concordance, and that it provides a lower bound for the `slice genus' of the colored link.

     

Integration of singular braid invariants and graph cohomology

We prove necessary and sufficient conditions for an arbitrary invariant of braids with double points to be the `` derivative' of a braid invariant. We show that the ``primary obstruction to integration' is the only obstruction. This gives a slight generalization of the existence theorem for Vassiliev invariants of braids. We give a direct proof by induction on which works for invariants with values in any abelian group.

We find that to prove our theorem, we must show that every relation among four-term relations satisfies a certain geometric condition. To find the relations among relations we show that of a variant of Kontsevich's graph complex vanishes. We discuss related open questions for invariants of links and other things.

     

On the singular braid monoid of an orientable surface

In this paper we show that the singular braid monoid of an orientable surface can be embedded in a group. The proof is purely topological, making no use of the monoid presentation.

     

<Emphasis Type=Italic>S</Emphasis>-graphs and braid index of links

We study the relationship between reduction operations on link diagrams and S-graphs associated with them. We are motivated by the problem of computing the braid index of a link and some well known conjectures concerning the braid index of a link and the writhe of its diagrams. Possible counterexamples are discussed in terms of both S-graphs and link diagrams. We also indicate the relation of S-graphs to singular links regarded up to an appropriate equivalence relation.     

Homogeneous links, Seifert surfaces, digraphs and the reduced Alexander polynomial

We give a geometric proof of the following result of Juhasz. Let a _g be the leading coefficient of the Alexander polynomial of an alternating knot K. If |a _g | <  4 then K has a unique minimal genus Seifert surface. In doing so, we are able to generalise the result, replacing ‘minimal genus’ with ‘incompressible’ and ‘alternating’ with ‘homogeneous’. We also examine the implications of our proof for alternating links in general.     

Virtual singular braids and links

Virtual singular braids are generalizations of singular braids and virtual braids. We define the virtual singular braid monoid via generators and relations, and prove Alexander- and Markov-type theorems for virtual singular links. We also show that the virtual singular braid monoid has another presentation with fewer generators.     

Primitive links of non-singular Morse-Smale flows on the special Seifert fibered manifolds

In this paper we consider a non-singular Morse-Smale flow Φ_t on an irreducible, simple, closed, orientable 3-manifold M. We define a primitive flow ψ_t from Φ_t, and call the link type of the closed orbits of ψ_t a primitive link of Φ_t. We show that the link types of the primitive links are finite and every non-singular Morse-Smale flow on M is obtained from a primitive flow by exchanging the flow in a regular neighborhood of attracting or repelling closed orbits.     

Maximal and extremal singular graphs

A graph G is singular if the nullspace of its adjacency matrix is nontrivial. Such a graph contains induced subgraphs called singular configurations of nullity 1. We present two algorithms. One is for the construction of a maximal singular nontrivial graph G containing an induced subgraph, which is a singular configuration with the support of a vector in its nullspace as in that of G. The second is for the construction of a nut graph, a graph of nullity one whose null vector has no zero entries. An extremal singular graph of a given order, with the maximal nullity and support, has a nut graph as a maximal singular configuration.     

Notes on the braid index of closed positive braids

The Morton–Franks–Williams inequality for a link gives a lower bound for the braid index in terms of the HOMFLY polynomial. Franks and Williams conjectured that for any closed positive braid the lower bound coincides with the braid index. In this paper, we show that the bound is achieved for a certain class of closed positive braids. We also give an infinite family of prime closed positive braids such that the lower bound does not coincide with their braid indices.     

Knots and links in spatial graphs

The main purpose of this paper is to show that any embedding of K₇ in three-dimensional euclidean space contains a knotted cycle. By a similar but simpler argument, it is also shown that any embedding of K₆ contains a pair of disjoint cycles which are homologically linked.     

Seifert surfaces in open books, and a new coding algorithm for links

We show that for any link L, there exists a Seifert surfacefor L that is obtained by successively plumbing flat annulito a disk D, where the gluing regions are all in D. This furnishesa new way of coding links. We also present an algorithm to readthe code directly from a braid presentation.     

On the null-spaces of acyclic and unicyclic singular graphs

For acyclic and unicyclic graphs we determine a necessary and sufficient condition for a graph G to be singular. Further, it is shown that this characterization can be used to construct a basis for the null-space of G.     

Affine distance-transitive graphs and classical groups

This paper finishes the classification of the finite primitive affine distance-transitive graphs by dealing with the only case left open, namely where the generalized Fitting subgroup of the stabilizer of a vertex is modulo scalars a simple group of classical Lie type defined over the characteristic dividing the number of vertices of the graph. All graphs that are found to occur are known.     

A note on regular isotopy of singular links

We show that two isotopic oriented 4-valent singular link diagrams with transverse intersections are regularly isotopic if and only if they have the same writhe and the same rotation number.

     

Edge-forwarding index of star graphs and other Cayley graphs

We present a technique for building, in some Cayley graphs, a routing for which the load of every edge is almost the same. This technique enables us to find the edge-forwarding index of star graphs and complete-transposition graphs.