Mathematics – Geometric Topology
Scientific paper
2003-11-29
Mathematics
Geometric Topology
23 pages. The second half of the title and the first half of the abstract are new, reflecting new material added in v2
Scientific paper
The multi-variable Alexander polynomial (in the form of Conway's potential function), when stripped of a redundant summand, is shown to be of the form \Nabla_L(x_1-x_1^{-1},..,x_m-x_m^{-1}) for some polynomial \Nabla_L over Z. The Conway polynomial \nabla_L(z) coincides with z\Nabla_L(z,..,z). The coefficients of \Nabla_L and of the power series \Nabla^*_L:=\Nabla_L/(\Nabla_{K_1}...\Nabla_{K_m}), where K_i denote the components of L, are finite type invariants in the sense of Kirk-Livingston. When m=2, they are integral liftings of Milnor's invariants \bar\mu(1..12..2) of even length, including, in the case of \Nabla^*_L, Cochran's derived invariants \beta^k. The coefficients of \Nabla_L and \Nabla^*_L are closely related to certain Q-valued invariants of (genuine) finite type, among which we find alternative extensions \hat\beta^k of \beta^k to the case lk\ne 0, such that 2\hat\beta^1/lk^2 is the Casson-Walker invariant of the Q-homology sphere obtained by 0-surgery on the link components. Each coefficient of \Nabla^*_L (hence of \nabla^*_L:=\nabla_L/(\nabla_{K_1}...\nabla_{K_m})) is invariant under TOP isotopy and under sufficiently close C^0-approximation, and can be extended, preserving these properties, to all topological links. The same holds for H^*_L and F^*_L, where H_L and F_L are certain exponential parameterizations of the two-variable HOMFLY and Kauffman polynomials. Next, we show that no difference between PL isotopy and TOP isotopy (as equivalence relations on PL links in S^3) can be detected by finite type invariants. These are corollaries of the fact that any type k invariant (genuine or Kirk-Livingston), well-defined up to PL isotopy, assumes same values on k-quasi-isotopic links.
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