Abstract. The universal sl 2 invariant of string links has a universality property for the colored Jones polynomial of links, and takes values in the -adic completed tensor powers of the quantized enveloping algebra of sl 2 . In this paper, we exhibit explicit relationships between the universal sl 2 invariant and Milnor invariants, which are classical invariants generalizing the linking number, providing some new topological insight into quantum invariants. More precisely, we define a reduction of the universal sl 2 invariant, and show how it is captured by Milnor concordance invariants. We also show how a stronger reduction corresponds to Milnor link-homotopy invariants. As a byproduct, we give explicit criterions for invariance under concordance and link-homotopy of the universal sl 2 invariant, and in particular for sliceness. Our results also provide partial constructions for the still-unknown weight system of the universal sl 2 invariant.
IntroductionThe theory of quantum invariants of knots and links emerged in the middle of the eighties, after the fundamental work of V. F. R. Jones. Instead of the classical tools of topology, such as algebraic topology, used until then, this new class of invariants was derived from interactions of knot theory with other fields of mathematics, such as operator algebras and representation of quantum groups, and revealed close relationships with theoretical physics. Although this gave rise to a whole new class of powerful tools in knot theory, we still lack a proper understanding of the topological information carried by quantum invariants. One way to attack this fundamental question is to exhibit explicit relationships with classical link invariants. The purpose of this paper is to give such a relation, by showing how a certain reduction of the universal sl 2 invariant is captured by Milnor invariants.Milnor invariants were originally defined by J. Milnor for links in S 3 [23,24]. Their definition contains an intricate indeterminacy, which was shown by N. Habegger and X. S. Lin to be equivalent to the indeterminacy in representing a link as the closure of a string link, i.e. of a pure tangle without closed components [8]. Milnor invariants are actually well-defined integer-valued invariants of framed string links, and the first non-vanishing Milnor string link invariants can be assembled into a single Milnor map µ k . See Section 2 for a review of Milnor string link invariants.Milnor invariants constitute an important family of classical (string) link invariants, and as such, their connection with quantum invariants has already been the subject of several works. The first attempt seems to be due to L. Rozansky, who conjectured a formula relating Milnor invariants to the Jones polynomial [29]. An important step was taken by Habegger and G. Masbaum, who showed explicitly in [10] how Milnor invariants are related to the Kontsevich integral. More recently, A. Yasuhara and the first author gave explicit formulas relating Milnor invariants to the HOMFLYPT polynomial [22].Key wor...