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Milnor's concordance invariants for knots on surfaces

### Micah Chrisman

Algebraic & Geometric Topology 22 (2022) 2293–2353
##### Abstract

Milnor’s $\overline{\mu }$–invariants of links in the $3$–sphere ${S}^{3}$ vanish on any link concordant to a boundary link. In particular, they are trivial on any knot in ${S}^{3}$. Here we consider knots in thickened surfaces $\mathrm{\Sigma }×\left[0,1\right]$, where $\mathrm{\Sigma }$ is closed and oriented. We construct new concordance invariants by adapting the Chen–Milnor theory of links in ${S}^{3}$ to an extension of the group of a virtual knot. A key ingredient is the Bar-Natan $\mathrm{Zh}$ map, which allows for a geometric interpretation of the group extension. The group extension itself was originally defined by Silver and Williams. Our extended $\overline{\mu }$–invariants obstruct concordance to homologically trivial knots in thickened surfaces. We use them to give new examples of nonslice virtual knots having trivial Rasmussen invariant, graded genus, affine index (or writhe) polynomial, and generalized Alexander polynomial. Furthermore, we complete the slice status classification of all virtual knots up to five classical crossings and reduce to $4$ (out of $92800$) the number of virtual knots up to six classical crossings having unknown slice status.

Our main application is to Turaev’s concordance group $\mathsc{𝒱}\mathsc{𝒞}$ of long knots on surfaces. Boden and Nagel proved that the concordance group $\mathsc{𝒞}$ of classical knots in ${S}^{3}$ embeds into the center of $\mathsc{𝒱}\mathsc{𝒞}$. In contrast to the classical knot concordance group, we show $\mathsc{𝒱}\mathsc{𝒞}$ is not abelian, answering a question posed by Turaev.

##### Keywords
virtual knots, knot concordance, Milnor's concordance invariants
##### Mathematical Subject Classification
Primary: 57K10, 57K12, 57K45