In a recent paper [1] Fiedler introduced a simple invariant for a knot K in a line
bundle over a surface F by means of a ‘small’ state-sum, which keeps a count of
features of the links resulting from smoothing each crossing of the projection of K
on F . The invariant takes values in a quotient of the integer group ring of H1 (F ).
Fiedler gives a number of applications of his general construction. In particular,
where K is a closed braid, and can thus be regarded as a knot in a solid torus V ,
his method gives an invariant of a braid β ∈ Bn in Z[H1 (V )] = Z[x±1 ] modulo
the relation xn = 1. This invariant depends only on the closure of the braid in
V and hence gives an invariant of β up to conjugacy in Bn . Its behaviour under
Birman and Menasco’s exchange moves has been used to help in detecting when
two braids may be related by such a move.
The purpose of this paper is to show how Fiedler’s invariant for a closed
ˆ
braid β can be found in terms of the Burau representation of β, and hence from
ˆ
the 2-variable Alexander polynomial of the link β ∪ A consisting of the closed
ˆ
braid β and its axis A. Its construction here from the Alexander polynomial can
be compared with methods which yield Vassiliev invariants of degree 1 in other
contexts, and suggests possible interpretations of Fiedler’s invariants as Vassiliev
invariants of degree 1 in the line bundle.
Having seen how the special case of Fiedler’s invariant is related to an Alexan-
der polynomial I finish the paper with a suggestion of extracting similar invariants
from the 2-variable Alexander polynomial of a more general 2-component link.
These might be regarded as degree 1 Vassiliev invariants of one component of the
link when considered as a knot in the complement of the other component. It
would be interesting to know if there was any similar state sum interpretation of
these invariants in the more general setting.