Abstract:
We introduce geometric formulas for several low degree knot and link invariants, expressing
them as signed counts of certain quadrisecants, i.e., lines meeting the link in four points.
We start by describing a simple unified construction using maps of configuration spaces and
elementary intersection theory. By building a compactification modeled on the Fulton–MacPherson
construction, we interpret these invariants directly as certain intersection numbers.
We then switch to an intriguing geometric interpretation for signs of quadrisecants. It turns
out that the signs are determined by cross-ratios of the four intersection points and their
tangent lines (identifying the locus of sign changes as certain hyperboloids of one sheet).
If time permits, in the end we will briefly discuss a graph complex governing these formulas
and conjectural formulas for higher degree invariants
Advisor: Prof. Michael Polyak