Equivariant triple intersections

Delphine Moussard (1)

1 Dipartimento di Matematica [Pisa]

Given a null-homologous knot $K$ in a rational homology 3-sphere $M$, and the standard infinite cyclic covering $\tilde{X}$ of $(M,K)$, we define an invariant of triples of curves in $\tilde{X}$, by means of equivariant triple intersections of surfaces. We prove that this invariant provides a map $\phi$ on $\Al^{\otimes 3}$, where $\Al$ is the Alexander module of $(M,K)$, and that the isomorphism class of $\phi$ is an invariant of the pair $(M,K)$. For a fixed Blanchfield module $(\Al,\bl)$, we consider pairs $(M,K)$ whose Blanchfield modules are isomorphic to $(\Al,\bl)$, equipped with a marking, {\em i.e.} a fixed isomorphism from $(\Al,\bl)$ to the Blanchfield module of $(M,K)$. In this setting, we compute the variation of $\phi$ under null borromean surgeries, and we describe the set of all maps $\phi$. Finally, we prove that the map $\phi$ is a finite type invariant of degree 1 of marked pairs $(M,K)$ with respect to null Lagrangian-preserving surgeries, and we determine the space of all degree 1 invariants of marked pairs $(M,K)$ with rational values.

Mots clés: Knot; Homology sphere; Equivariant intersection; Alexander module; Blanchfield form; Borromean surgery; Null-move; Lagrangian-preserving surgery; Finite type invariant.; Finite type invariant

Domaines: Topologie géométrique [math.GT]; Topologie algébrique [math.AT]

Delphine Moussard. Equivariant triple intersections. 2014. ⟨hal-00954767v1⟩